Advances in Child Development and Behavior Volume 30

ADVANCES IN CHILD DEVELOPMENT AND BEHAVIOR

Volume 30

Contributors to This Volume Karen Adolph

David C. Geary

Lorraine E. Bahrick

Erica E. Kleinknecht

Patricia J. Bauer

Robert Lickliter

L. Beckwith

Lonna M. Murphy

Thomas J. Berndt

Amanda Sheffield Morris

Melissa M. Burch

A. Rozga

Nancy Eisenberg

M. Sigman

ADVANCES IN CHILD DEVELOPMENT AND BEHAVIOR

edited by Robert V. Kail Department of Psychological Sciences Purdue University West Lafayette, Indiana

Volume 30

ACADEMIC PRESS An imprint of Elsevier Science Amsterdam Boston London New York Oxford Paris San Diego San Francisco Singapore Sydney Tokyo

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Contents Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Learning to Keep Balance KAREN A D O L P H I. II. III. IV.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Learning to Keep Balance: Sway Model of Balance Control . . . . . . . . . . . . . . . . . . . . . . . . . Flexibility and Specificity of Motor Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . How Development May Constrain Motor Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 3 9 27 36

Sexual Selection and Human Life History D AVID C. GEARY I. II. III. IV. V. VI.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Natural Selection and Life History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sexual Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Life History and Sexual Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Human Developmental Sex Differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41 42 51 56 63 87 89

Developments in Early Recall Memory: Normative Trends and Individual Differences PATRICIA J. BAUER, MELISSA M. BURCH, AND ERICA E. K L E I N K N E C H T I. II. III. IV. V. VI.

Initiating the Study of Early Recall Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterizing Recall Memory in the First Two Years of Life . . . . . . . . . . . . . . . . . . . . . . . . Individual Differences in Long-Term Recall: Children's Gender, Children's Language Proficiency, and Variability in Initial Learning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Individual Differences in Long-Term Recall: Children's Temperament Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Children's Temperament and Mothers' Language as Interacting Sources of Individual Differences in Long-Term Recall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 110 115 121 130 138 146

Contents

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Intersensory Redundancy Guides Early Perceptual and Cognitive Development LORRAINE E. B A H R I C K AND R O B E R T LICKLITER I. II. III. IV. V. VI.

Introduction: Historical Overview and Perspectives on Perceptual Development .... Amodal Relations and the Multimodal Nature of Early Experience . . . . . . . . . . . . . . . . . . Unimodal-Multimodal Dichotomy in Developmental Research . . . . . . . . . . . . . . . . . . . . . . Neural and Behavioral Evidence for Intersensory Interactions . . . . . . . . . . . . . . . . . . . . . . . Intersensory Redundancy Hypothesis: Toward an Integrated Theory of Perceptual Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and Directions for Future Study of Perceptual Development . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

153 156 158 159 163 178 181

Children's Emotion-Related Regulation NANCY EISENBERG AND A M A N D A SHEFFIELD MORRIS I. II. III. IV. V. VI. VII. VIII.

Definition of Emotion-Related Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Brief Review of Views of Emotion Regulation in Theories of Emotion . . . . . . . . . . . Conceptual Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Age-Related Trends in Emotion-Relevant Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement of Emotion Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relation of Emotion-Relevant Regulation to Quality of Social Functioning . . . . . . . . . Relations of Dispositional Resiliency to Effortful and Reactive Control and Socioemotional Functioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and Future Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

190 191 194 198 204 210 215 219 220

Maternal Sensitivity and Attachment in Atypical Groups L. BECKWITH, A. ROZGA, AND M. SIGMAN I. II. III. IV. V. VI.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organization of Attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maternal Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of Child Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atypical Groups of Mothers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

231 233 236 242 255 260 263

Influences of Friends and Friendships: Myths, Truths,

and Research Recommendations

THOMAS J. BERNDT AND L O N N A M. M U R P H Y I. II. III.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influences of Friends' Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influences of Friendship Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

275 277 294

Contents

IV. V.

Influences of Friends' Characteristics in Friendships Differing in Quality . . . . . . . . . . . Conclusions and Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Contents of Previous Volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Contributors Numbers in parentheses indicate the pages on which the authors' contributions begin.

KAREN ADOLPH

Department of Psychology, New York University, New York, New York 10003 (1) LORRAINE E. BAHRICK

Department of Psychology, Florida International University, Miami, Florida 33199 (153) PATRICIA J. BAUER

Institute of Child Development, University of Minnesota, Minneapolis, Minnesota 55455 (103) L. BECKWITH

Department of Pediatrics, University of California at Los Angeles, Los Angeles, California 90024 (231) THOMAS J. BERNDT

Department of Psychological Sciences, Purdue University, WestLafayette, Indiana 47907 (275) MELISSA M. BURCH

Institute of Child Development, University of Minnesota, Minneapolis, Minnesota 55455 (103) NANCY EISENBERG

Department of Psychology, Arizona State University, Tempe,Arizona 85287 (189) DAVID C. GEARY

Department of Psychological Sciences, University of Missouri at Columbia, Columbia, Missouri 65211 (41) ERICA E. KLEINKNECHT

Institute of Child Development, University of Minnesota, Minneapolis, Minnesota 55455 (103) ROBERT LICKLITER

Department of Psychology, Florida International University, Miami, Florida 33199 (153) LONNA M. MURPHY

Department of Psychological Sciences, Purdue University, WestLafayette, Indiana 47907 (275)

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Contributors

AMANDA SHEFFIELD MORRIS

Department of Psychology, University of New Orleans, New Orleans, Louisiana 70148 (189) A. ROZGA

Department of Psychiatry, University of California at Los Angeles, Los Angeles, California 90024 (231) M. SIGMAN

Department of Psychiatry, University of California at Los Angeles, Los Angeles, California 90024 (231)

Preface The amount of research and theoretical discussion in the field of child development and behavior is so vast that researchers, instructors, and students are confronted with a formidable task in keeping abreast of new developments within their areas of specialization through the use of primary sources as well as being knowledgeable in areas peripheral to their primary focus of interest. Moreover, journal space is often simply too limited to permit publication of more speculative kinds of analyses that might spark expanded interest in a problem area or stimulate new modes of attack on a problem. The serial publicationAdvances in Child Development and Behavior is intended to ease the burden by providing scholarly technical articles serving as reference material and by providing a place for publication of scholarly speculation. In these documented critical reviews, recent advances in the field are summarized and integrated, complexities are exposed, and fresh viewpoints are offered. These reviews should be useful not only to the expert in the area but also to the general reader. The series is not intended to reflect the development of new fads, and no attempt is made to organize each volume around a particular theme or topic. Manuscripts are solicited from investigators conducting programmatic work on problems of current and significant interest. The editors often encourage the preparation of critical syntheses dealing intensively with topics of relatively narrow scope but of considerable potential interest to the scientific community. Contributors are encouraged to criticize, integrate, and stimulate, but always within a framework of high scholarship. Although appearance in the volumes is ordinarily by invitation, unsolicited manuscripts will be accepted for review. All papers--whether invited or submitted--receive careful editorial scrutiny. Invited papers are automatically accepted for publication in principle, but usually require revision before final acceptance. Submitted papers receive the same treatment, except they are not automatically accepted for publication even in principle and may be rejected. I acknowledge with gratitude the aid of my home institution, Purdue University, which generously provided time and facilities for the preparation of this volume. I also thank David F. Bjorklund for his editorial assistance. Robert V. Kail

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LEARNING

TO KEEP BALANCE

Karen E.

Adolph

DEPARTMENT OF PSYCHOLOGY NEW YORK UNIVERSITY NEW YORK, NY 10003

I. INTRODUCTION A. THE IMPORTANCE OF B A L A N C E B. OVERVIEW II. LEARNING TO KEEP BALANCE: SWAY M O D E L OF B A L A N C E C O N T R O L A. REGION OF PERMISSIBLE P O S T U R A L SWAY B. L O C A L VARIABILITY AFFECTS THE SIZE OF THE SWAY REGION C. D E V E L O P M E N T A L CHANGES AFFECT DEFINING PARAMETERS III. FLEXIBILITY AND SPECIFICITY OF M O T O R L E A R N I N G A. SITTING AND C R A W L I N G AT THE EDGE OF GAPS B. C R A W L I N G AND WALKING DOWN SLOPES C. LEARNING TO DETECT THREATS TO BALANCE: THE VISUAL CLIFF AND OTHER FALLING TASKS IV. HOW D E V E L O P M E N T M A Y CONSTRAIN M O T O R LEARNING A. CONTENT OF EVERYDAY EXPERIENCE B. ENSURING FLEXIBILITY AND SPECIFICITY C. CONCLUSION REFERENCES

I. I n t r o d u c t i o n A. THE I M P O R T A N C E OF B A L A N C E

Motor development is a delicate balancing act. Newborn infants are slaves to the pull of gravity, but by 2 months babies can lift their heads from the crib mattress, balance their heads between their shoulders, and turn to look at an interesting event. By 6 months, they can balance in a sitting position anchored to the floor with their outstretched legs, lean forward to retrieve a fallen toy, and use their hands to play with objects or clap. By 8 months, infants can balance on hands and knees, crawl

ADVANCESIN CHILDDEVELOPMENT AND BEHAVIOR,VOL.30

Copyright 2002, Elsevier Science (USA). All rights reserved.

0065-2407/02 $35.00

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Karen E. Adolph

across the living room floor, steer around furniture, and clamber over objects in their path. By 10 months they can pull themselves to a wobbly stand and walk sideways hanging onto the coffee table or couch for support. By the end of their first year, infants can balance on two feet; walk independently across the room; crouch down to peer under the table; stretch upward to pull books off the shelves; stop, start, and turn comers; and modify their step length and walking speed. These homely postural accomplishments are the stuff of motor development. Students' developmental textbooks are graced with a requisite chart of motor milestones. Parents' home videos and photo albums highlight the postural milestones of their infants' first year. In fact, the pioneering researchers of the 1930s and 1940s were so captivated by infants' dramatic transformation from worm to person and by the seeming regularity with which the metamorphoses occurred that the field of motor development was founded on normative descriptions of the ages and stages that characterize the various postural milestones. Although modem researchers no longer focus on cataloging infants' postural milestones, they agree that the most basic motor control problem is maintaining balance (Reed, 1982). Balance is not only important for relatively stationary positions such as sitting and standing, but also it provides the necessary stable base to support movements of the head, torso, or limbs. Everyone who has broken a rib or thrown out his or her back has experienced the centrality of posture for controlling movements--lifting an arm from the bed to grasp a glass of water becomes a painful lesson about the muscles involved in stabilizing the torso, and lifting a leg to put pants on or to climb stairs is nearly impossible. In the laboratory, researchers have shown that participants' abdominal muscles and back muscles fire prior to a reaching movement with the arms, indicating that postural stabilization is a primary part of the motor plan (Hofsten, 1993). Stabilizing the body and keeping it in balance are prerequisites for adaptive control of movement. B. OVERVIEW Two stories are woven throughout this chapter. The first story concerns the central issue in motor control of maintaining balance, but I approach the problem as a psychologist rather than as a biomechanist. ! argue that a complete understanding of balance control involves much more than a description of muscles, torques, and lever arms. Such understanding involves the basic psychological functions of perceiving the layout, discovering one's own capabilities, and forming plans to accomplish one's goals (Gibson & Pick, 2000; Gibson, 1958, 1979). Most important, I provide evidence that infants must learn to keep their bodies in balance and that this most basic and practical kind of knowledge represents an important and sophisticated psychological achievement. Learning to avoid falling down requires immense psychological flexibility for coping with moment to moment variations in infants' bodies, environments, and goals. In addition, I incorporate these arguments about balance control into a new account of a classic phenomenon--infants'

Learning to Keep Balance

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avoidance on a visual cliff and provide a unifying framework for understanding a myriad of other tasks in which babies must avoid falling down. The second story concerns a central issue in developmental psychology--the relation between learning and development. I describe how infants' learning about balance control is nested in the context of ongoing developmental changes in their bodies, skills, and environments. Moreover, I argue that peripheral developmental changes may constrain the course of learning to ensure that infants acquire the requisite behavioral flexibility and specificity that they need to respond adaptively under continually changing local conditions. The chapter is divided into three sections. In the first section, I present a sway model of balance control proposed by me and my colleagues (Adolph, 2000; Adolph & Avolio, 2000; Adolph & Eppler, 1998, 2002, Adolph, Eppler, Marin, Weise, & Clearfield, 2000). According to our model, learning about balance control underlies adaptive responses to a cliff or any other risky ground surface or task that poses potential threats to balance. In the second section, I describe several experiments that provide evidence for the sway model. The weight of the evidence rests on the flexibility and specificity of infants' motor learning. On the sway model, infants' responses should be flexible enough to cope with changes in their bodies, skills, and task constraints but specific to each postural milestone in development. The final section addresses a central theme of the chaptermthe relation between learning and development. I will conclude with some suggestions for how developmental changes can help to ensure that infants' learning has just the right amount of flexibility and specificity to promote adaptive responding in a variable world.

II. Learning to Keep Balance: Sway Model of Balance Control A. REGION OF PERMISSIBLE POSTURAL SWAY

Keeping balance appears so effortless in adults' everyday actions and so difficult in babies' attempts to perform the same kinds of skills that a casual observer might think that adults have solved the problem of balance control but that babies have not. In fact, we never outgrow the problem of keeping balance. The process of imminent balance loss and recovery is continual and cyclical. Figure 1A shows a simple physical model of balance control in quiet, upright stance. The body is represented by a mass, M, along an inverted pendulum with the pivot point at the ankles. The dashed lines represent a three-dimensional cone-shaped region in which the body gently sways and teeters over its base of support. The angular displacement of the body as it sways back and forth is represented by tO. (The swaying motions are exaggerated in the figure to illustrate the point more clearly). To keep themselves from falling, infants must maintain their bodies within the cone-shaped region, termed the "cone of reversibility" (McCollum & Leen,

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Karen E. Adolph

Fig. 1. Physical model of balance control in upright stance on (A) flat ground, (B) with increased mass and displaced center of mass, and (C) on a sloping surface. M is Mass; tO is angular distance body can sway without falling. Dashed lines represent the region of permissible postural sway.

1989) or the "region of permissible postural sway" (Adolph, 2000; Adolph & Eppler, 1998; Riccio, 1993; Riccio & Stoffregen, 1988; Stoffregen & Riccio, 1988). Infants fall if their bodies move outside the sway region without sufficient muscle torque to pull themselves back into position. Thus, a sway in one direction must be counteracted by a compensatory sway in the opposite direction. Such nearly imperceptible oscillations of the body are revealed when adults and infants maintain quiet stance on a force plate or in a "moving room" where the walls move to simulate the optic flow normally generated by postural sway (Bertenthal & Bai, 1989; Lee & Lishman, 1975; Nashner & McCollum, 1985; Stoffregen, Schmuckler, & Gibson, 1987). The size of the sway region, that is, the size of | depends on the amount of muscle torque infants can generate relative to the size of destabilizing torque. (Muscle torque is the amount of angular force generated by the body and destabilizing torque is the amount of force trying to pull the body over.) In the dynamic postures of locomotion, the balance control problem is similar, only now the sway region shifts its position in space as if undergoing a series of deliberate, controlled near-falls. In crawling and walking, for example, the base of support shifts forward in anticipation of catching the body as the moving limb swings forward from step to step (Breniere & Bril, 1988; Goldfield, 1989).

Learning to Keep Balance

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B. LOCALVARIABILITYAFFECTSTHE SIZE OF THE SWAYREGION What makes balance control a psychological problem, not just a biomechanical one, is that the size of the sway region is continually changing. Figures 1B and 1C show two causes of change in the size of the sway region: changes in functional body dimensions and changes in the ground surface. In Figure 1B, the stick figure is loaded with additional mass and the location of the center of mass is raised along the lever arm. Both factors--increased mass and displacing the center of mass away from the pivot point--increase the size of destabilizing torques and decrease the size of the sway region before falling over. Adults experience such changes in their functional body dimensions when they walk carrying a heavy backpack. In Figure 1C, the stick figure is perched on a slope. Uptight balance is more precarious on a downward slope than on flat ground because the size of the sway region is smaller. The feet are at an angle, decreasing the base of support. Available muscle torque is diminished because leg and trunk muscles must do extra work to keep the body perpendicular to gravity and to curb forward momentum. The vertical distance that the body falls increases during each step. The critical point for my argument is that nearly everything changes the size of the sway region. The ground surface is typically variable in terms of slant, elevation, friction, rigidity, and obstacles. The body's functional dimensions are typically variable due to carrying loads or shifts in the location of the center of mass. Simply raising the arms, bending the knees, or tilting the head forward change the location of the center of mass! In addition, because the parts of the body are mechanically linked, movements themselves affect balance control. In fact, the size of the sway region fluctuates from step to step, with ever,,, irregularity in terrain, with every change in the location of the center of mass due to body movements or shifting a load, and so on. Infants would have to track these changes on a moment to moment basis so that keeping their bodies inside the region of reversible postural sway would require a continual process of calibration in real time. C. DEVELOPMENTALCHANGESAFFECTDEFININGPARAMETERS 1. Development Creates Local Variability The problem of maintaining balance is even more complicated because of ongoing developmental changes. One way that development affects balance is by creating local variability. In particular, developmental changes in infants' body dimensions, skill level, and exposure to environmental properties all affect the size of the sway region. Infants' bodies are ill suited for balance control. Their short bodies sway faster than adults' bodies and require quicker corrections, much like trying to balance a ruler upright on the palm of your hand versus trying to balance a yardstick

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Karen E. Adolph

(Forssberg & Nashner, 1982; McCollum & Leen, 1989). Moreover, infants' bodies are top-heavy due to their relatively large heads and trunks and short legs (Palmer, 1944). Like a top-heavy bookcase threatening to tip, infants' top-heavy dimensions make them less stable during stance and locomotion (Adolph & Eppler, 2002). In addition to their precarious dimensions, the size and shape of infants' bodies are rapidly changing. The rapid growth and redistribution of body mass during the first 2 years of life rival the dramatic changes during the fetal period and puberty (Palmer, 1944). Moreover, growth changes in infancy are abrupt rather than continuous. Infants literally grow longer and heavier in the course of a day (e.g., 0.5-2.0 cm) separated by plateaus of no change for weeks at a stretch (Lampl, 1993; Lampl, Veldhuis, & Johnson, 1992). Thus, babies often wake up to a differently sized sway region with different constraints on maintaining balance. Changes in infants' body dimensions co-occur with dramatic improvements in their level of motor skill. Many infants begin crawling using idiosyncratic combinations of arm and leg movements while scraping or hopping along with their bellies on the floor. Later, they discover how to crawl on hands and knees or hands and feet. Across all of their crawling styles, infants display rapid improvements in their level of crawling skill. Crawling speed, for example, increases by 720% over the first 20 weeks of crawling (Adolph, Vereijken, & Denny, 1998), and the size of infants' crawling steps increases by 265%. Walking skill proceeds from a Charlie Chaplin (arms supinated upward and toes pointing outward) or Frankenstein gait (stiff legs and outstretched arms) to the bouncing walk of a typical toddler. The size of infants' walking steps increases by 137% over the first 16 weeks of walking and their base of support--the lateral distance between their feet--decreases by 150% (Adolph, Vereijken, & Shrout, 2002). Dramatic changes in skill levels imply changing ability to cope with destabilizing torque. Finally, changing bodies and skill levels co-occur with developmental changes in the environments in which infants travel. Each week after the onset of independent mobility, infants are exposed to new surfaces (grass, sand, cement, shag rug, linoleum, etc.) and obstacles (Chan, Lu, Marin, & Adolph, 1999; Chan, Biancaniello, Adolph, & Marin, 2000). Parents may implicitly take more mature body dimensions and more advanced skill levels into account when they allow their babies freer access to new features of the environment such as stairs, slopes, and playground climbers. As infants acquire more skill, they may be motivated and capable of exploring new aspects of the terrain. Thus, peripheral developmental changes--that is, changes which do not involve the central nervous system--are central to understanding the problem of balance control. Unstable body proportions, sudden growth changes, dramatic improvements in skill level, and exposure to an ever-widening array of environmental opportunities for interaction all result in changes in the size of the sway region. Variability in the size of the sway region, in turn, requires continual monitoring and updating for maintaining balance.

Learning to Keep Balance

7

2. Development Creates New Perception-Action Systems A second way that developmental changes affect balance control is by creating new perception-action systems with new defining control parameters. The series of postural milestones that characterize motor development differ in more than the size of the relevant sway regions; they differ in the location of the sway regions and the key parameters that define them. Figure 2 shows three major postural milestones in motor developmentmsitting, crawling, and walking. In a sitting posture, infants are anchored to the floor by their outstretched legs. In a crawling posture, they move forward while balancing on hands and knees. And, in a walking posture, they move forward while maintaining balance over their feet. Most infants sit at about 6 months of age, crawl at 8 months, and walk at 12 months. However, the ages and order of milestones are not important. The critical point is that infants do not acquire all of these skills at the same time. Sitting, crawling, and walking postures appear staggered over many months of development. The sway model proposes that each postural milestone in development is actually a different balance control system with different relevant control parameters. Sitting, crawling, and walking involve different regions of permissible postural sway for different key pivots around which the body rotates. The hips are typically the key pivot in a sitting posture, the wrists in a crawling posture, and the ankles in an uptight posture. Moreover, each balance control system involves different muscle groups for generating compensatory sway; different vantage points for viewing the ground ahead; different correlations between visual, vestibular, and somatosensory information; and different limbs are available for exploring the ground surface and obstacles. Sitting, crawling, and walking are quite different perception-action systems.

3. Consequencesfor Learning and Development New balance control systems and ongoing developmental changes make particular demands on learning. On the sway model, infants must learn to gauge their region of permissible postural sway. This learning entails two steps: first,

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8

Karen E. Adolph

identification of control parameters, and second, calibration of those parameters in real time. Each time that infants acquire a new posture in development, they must learn to identify the relevant control parameters for keeping balance: What is the key pivot? What muscle groups control body sway around the pivot? What perceptual information is relevant? Only through experience fighting gravity in a new posture, can babies identify the location of the new sway region and the defining control parameters for the new balance control system. Once infants identify the relevant parameters, they must learn to calibrate the settings of the relevant parameters in real time. Thus, the second step of learning to keep balance is to tweak the parameters of the new balance control system to cope with the continually changing size of the new angle of permissible postural sway. A central prediction of the sway model is that learning should be both highly specific and extremely flexible. The dual processes of identification and calibration correspond to specificity and flexibility of learning, respectively. Transitions to new postures in development create specificity of learning. On the sway model, because developmental changes in posture create new balance control systems, learning to identify the relevant control parameters should be specific to each balance control system in development. At the same time, changes in the size of the sway region demand flexibility of learning. Within each postural milestone in development, learning should be flexible enough to cope with continual changes in body, skill level, and task--the everyday moment to moment changes in the size of the sway region. Thus, as shown in Figure 3, the sway model predicts that infants should display separate learning curves over days of experience with each posture and that there would be no transfer across developmental changes in posture. The adaptiveness of infants' responses in novel predicaments that challenge balance control should be related to the duration of their everyday experience keeping balance in the test posture. The discrepancy between infants' responses in each posture should depend on the relative timing of their onset and the slope of the learning curves. That is, novice sitters, crawlers, and walkers should display a high rate of errors; they

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Learning to Keep Balance

9

should misjudge their abilities, lose balance, and fall over. In contrast, experienced sitters, crawlers, and walkers should respond adaptively to novel changes in their bodies, skills, and task constraints. With experience, responses should become increasingly scaled and fine-tuned to the relative degree of risk. In the weeks before learning reaches asymptote, infants should show more adaptive responses in familiar postures compared with unfamiliar ones.

HI. Flexibility and Specificity of Motor Learning In a series of experiments, we tested the predictions of the sway model by comparing infants' responses to risky ground surfaces in more versus less experienced postures (Adolph, 1997, 2000; Adolph, Eppler, & Gibson, 1993a; Leo, Chiu, & Adolph, 2000). We tested infants in novel tasks--stretching over gaps in the surface of support and descending slopes--so that we could observe how they decided in real time whether a surface posed a potential threat to balance. We used a graded array of surfaces--small and large gaps, shallow and steep slopes--presenting varying amounts of risk so that we could measure the adaptiveness of infants' decisions quite precisely. A. SITTINGAND CRAWLINGAT THE EDGE OF GAPS 1. Developmental Design and Rationale

In the "gaps" studies, we tested flexibility and specificity of infants' knowledge about balance control at the edge of a gap in the surface of support (Adolph, 2000). Infants were tested in two postures--sitting and crawling--in the same test session. In both postures, they were perched at the edge of an adjustable gap---a veritable cliff. Their task was the same in both postures. We encouraged them to span the gap by leaning forward while extending an arm toward an attractive toy. We disentangled infants' age from their sitting and crawling experience using an age-matched control design. All the infants were the same age, but their experience varied freely. The experimental design capitalized on the fact that infants display a period of overlap between sitting and crawling milestones. Typically, infants begin sitting many weeks before they begin crawling, meaning that they have much more experience with the sitting posture compared with the crawling posture. Nineteen infants participated. All were 9 months old ( 4-1 week) and all had more experience keeping balance in a sitting posture (M = 3.4 months of sitting experience) compared with a crawling posture (M -- 1.5 months of crawling experience). We reasoned that if experience maintaining balance promotes flexible, adaptive responding in novel tasks that challenge balance, then infants should respond adaptively in the novel gaps task by stretching their arms over safe gaps but refusing to attempt risky ones. If experience is specific to each posture in development, then

10

Karen E. Adolph

infants should show more adaptive responses in their more experienced sitting posture, even when tested on the same apparatus in the same task in the same session.

2. GapsApparatus The most serious threat to balance is a sheer drop-off because there is no floor to support the body. We tested infants' ability to respond adaptively to a drop-off using a modem variant of the classic visual cliff paradigm (Gibson & Walk, 1960; Walk, 1966; Walk & Gibson, 1961). In the classic experimental arrangement, babies are tested on a visual cliff rather than on a real one to ensure their safety. Infants begin on a centerboard dividing a large glass table. On the "deep" side, the apparatus looks like a sheer drop-off because the floor is visible far below the invisible safety glass. On the "shallow" side, the apparatus looks like solid ground because the floor tiles are attached to the underside of the glass. Mothers call to their infants from first one side, then the other. Infants must decide whether to crawl over the apparent drop-off or avoid going. Like the visual cliff, our gaps apparatus (shown in Figure 4) presented a dropoff in the surface of support. However, the gaps apparatus differed from the visual

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Fig. 4. Gaps apparatus and procedure in (A) the sitting condition with movable stick in Experiment 1, (B) the crawling condition in Experiments I and 2, and (C) the sitting condition with movable plaO~orm in Experiment 2. Experimenter (shown)followed alongside infants to ensure their safety. Parents (not shown) stood at the far side of the landing pla(form and encouraged infants' efforts. From "Specificity of Learning: Why Infants Fall Over a Veritable Cliff" by K. E. Adolph, 2000, Psychological Science, 11(4), p. 292, Copyright 2000 by the American Psychological Society. Reprinted with permission.

Learning to Keep Balance

11

cliff in several important ways. First, the visual cliff is actually perfectly safe for locomotion because it is covered in safety glass. Infants determine this in the course of testing and, although they continue to be leery of crossing the glass by patting it, gazing down into the precipice, and clinging to the supporting walls of the apparatus, avoidance responses actually attentuate over repeated trials (Campos, Hiatt, Ramsay, Henderson, & Svejda, 1978; Eppler, Satterwhite, Wendt, & Bruce, 1997; Titzer, 1995). As a result, researchers are limited to one or two trials per infant and infants cannot be tested longitudinally. Findings must be reported in terms of the proportion of infants who avoided the apparent drop-off. In contrast, on the gaps apparatus, the floor had an actual hole with real consequences for balance control. Instead of safety glass, a vigilent experimenter spotted infants to ensure their safety, catching them as they fell. Infants found it aversive to feel themselves falling downward and thus were highly motivated to avoid gaps they perceived to be risky. As a result, we could present infants with 50-100 trials in a single test session. Using a crude but robust psychophysical procedure, we could draw response curves for each infant. A second important difference between the gaps apparatus and the visual cliff concerns the availability of multimodal perceptual information. On the visual cliff, visual and haptic information are in conflict. The visual cliff looks risky but feels safe. The discrepancy in perceptual information makes it difficult to interpret infants' responses. On the gaps apparatus, visual and haptic information are in agreement. Large gaps look risky, feel risky, and are risky. Finally, the visual cliffhas only two settings--the deep and the shallow sides. As with discrepant visual information, this factor also renders it difficult to interpret infants' avoidance responses. The deep side of the visual cliff would be impossibly risky, of course, without the safety glass. But so would many gradations of dropoff that present far less of a compelling visual display. For a crawling infant, a 3-ft precipice is no more risky than a 2-ft precipice: The probability'of crawling successfully is zero in both cases. If avoidance on the deep side reflects a motor decision about whether crossing is feasible, then infants should avoid smaller but equally risky increments as often as the larger more perceptually salient ones. To address this issue, the gaps apparatus was adjustable so that we could present infants with a nearly continuous gradation of gap sizes. The gaps apparatus consisted of a stationary starting platform (106-cm long • 76-cm wide • 86-cm high) and a movable landing platform (157-cm long x 76-cm wide • 86-cm high). In contrast to the visual cliff, we kept the vertical distance to the floor constant and varied the distance across the precipice. By rolling the landing platform along a calibrated track, we could create a gap between the two platforms varying from 0-90 cm in 2-cm increments. The largest gap size was similar to the standard visual cliff along all three dimensions. At the smallest gap distances, balance was trivial. At intermediate gap sizes, infants had to gauge the necessary forces required to span the gap. At the largest gap sizes, the distance exceeded infants' limit of permissible

12

Karen E. Adolph

postural sway. As on the visual cliff, avoidance was the appropriate response to impossibly large gaps. 3. Staircase Procedure to Normalize Risk Level

In the sitting condition (shown in Figure 4A), infants were placed in a sitting position at the edge of the starting platform with their legs dangling into the gap. They were encouraged to lean forward and extend their arms out over the gap to retrieve a toy. The lure was attached by velcro to the end of a stick protruding through a box which rested on the landing platform. An assistant moved the stick forward and backward to create gaps of 0-90 cm between the toy and the edge of the starting platform. Gap distance was varied by moving the stick rather than the landing platform to prevent pinching infants' legs in the gap and to keep them from propping their feet or free hand on the far side of the gap to aid in balance control. In the crawling condition (shown in Figure 4B), infants were placed on their hands and knees in the middle of the starting platform. They were encouraged to crawl to the edge of the gap, lean forward, and extend an arm toward the landing platform as they crawled over the gap. Toys on the landing platform provided the incentive to span the gap. In both conditions, parents waited at the far end of the landing platform and encouraged their babies to retrieve the toys. An experimenter (shown in Figure 4) followed closely alongside infants to ensure their safety but did not provide hands-on support unless infants fell into the gap. Both platforms were covered in soft carpet and the interior of the crevasse was lined with foam as an additional safety precaution. Because infants of the same age have widely varying body dimensions and levels of motor skill, we used a modified psychophysical staircase procedure to normalize the definition of safe and risky gaps relative to each infant's body size and skill in each condition (Adolph, 1995, 1997, 2000; Adolph & Avolio, 2000). The staircase procedure is a classic method in psychophysics for estimating a perceptual threshold using a minimal number of trials (Cornsweet, 1962). When estimating a perceptual threshold, researchers plot the function spanning the increments when the observer is 100% accurate to those when the observer is guessing at 50% chance levels. In this case, we modified the classic staircase procedure to estimate a "motor threshold." The function spanned the increments of gap size where infants spanned the gap successfully on 100% of the trials to those when success rates dropped to 0%. Trials were coded in real time as either successful attempts (safely spanned the gap), failed attempts (fell into the gap), or refusals (avoided the gap). For the purpose of estimating infants' gap thresholds, we treated failed attempts and refusals as equivalent, unsuccessful outcomes. Infants began with an easy baseline gap (10 cm in the sitting condition and 4 cm in the crawling condition). After successful trials, the experimenter increased the gap size by 6 cm. The experimenter repeated trials after one failure or refusal and decreased gap size by 4 cm

Learning to Keep Balance

13

after two consecutive unsuccessful trials. Easy baseline increments were interspersed throughout the protocol to maintain infants' motivation to span the gap. The procedure continued until converging on a gap threshold with a >67% success criterionmthe largest gap that infants spanned successfully at least two out of three times and less than two out of three times at the next 2-, 4-, and 6-cm increments. (Note that gap thresholds were not calculated using curve fitting or linear interpolation as is customary in perceptual psychophysics. Thus, with the >67% success criterion, infants sometimes displayed >67% successes at their estimated gap thresholds and success rates often dropped off sharply near to 0% successes on all larger increments of gap.) Gap thresholds varied widely between infants and conditions. The range in gap thresholds was 20-32 cm for sitting and 2-18 cm for crawling. In the sitting condition, the probability of spanning the gap successfully decreased from .96 at the gap threshold to .07 at gap distances 12 cm larger than threshold. In the crawling condition, the probability of success decreased from .86 at the gap threshold to .06 at gaps 12 cm larger. By definition, infants' gap thresholds marked the difference between safe and increasingly risky gaps. The wide range in gap thresholds highlights the importance of normalizing the definition of risk to individual babies' skill level in each condition to allow comparisons between sitting and crawling postures and between infants with different gap thresholds.

4. Flexibility and Specificity in Sitting and Crawling Postures After identifying the gap threshold, the experimenter tested the accuracy of infants' motor decisions by presenting them with two probe trials at each of several safe and risky gaps (gaps 6 cm smaller than gap threshold, 6 cm larger than gap threshold, 12 cm larger than gap threshold, 18 cm larger than gap threshold, and the largest 90-cm gap). In total, infants received 17-42 trials in the sitting condition and 21-38 trials in the crawling condition. The important psychological question was whether infants could detect potential threats to balance on risky gaps, that is, whether infants' motor decisions were scaled to their gap thresholds. We indexed the accuracy of infants' motor decisions with an "attempt ratio" by calculating the ratio of infants' attempts to span the gap to the total number of trials: (successes + failures)/(successes + failures + refusals). We calculated an attempt ratio for each infant at each increment of gap in each condition. The ratio must be >.67 at the gap threshold by definition but can vary freely from 0 to 1 at all other increments of gap size. Our logic was that infants would have a high attempt ratio on gaps they determined to be safe and a low attempt ratio on gaps they perceived to be risky.

a. Adaptive Responding in Experienced Posture. As predicted by the sway model, infants showed impressive flexibility in their experienced sitting posture by

14

Karen E. Adolph

adapting their responses to the constraints of the novel gaps task (see dashed line in Figure 5A). In their sitting posture, infants attempted to span safe gaps but refused to attempt increasingly risky ones. Attempt ratios were uniformly at zero on the largest 90-cm increment. Apparently, uneventful, everyday experience coping with balance in a sitting position facilitated coping with threats to balance at the edge of the gaps in the laboratory task. None of the infants had experienced a serious fall at home nor were their responses related to experiencing minor falls at home. Moreover, infants rarely fell on risky gaps (only 19% of risky sitting trials). At most risky sitting trials, infants refused to span the gaps outright; they were no more likely to refuse after falling on an earlier trial than after succeeding on an earlier trial.

b. Falling in Unfamiliar Posture. Also as predicted by the sway model, infants showed striking specificity of knowledge in their unfamiliar crawling posture by

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Learning to Keep Balance

15

consistently misjudging constraints on keeping balance. Infants attempted impossibly risky gaps in the crawling condition on trial after trial, requiring rescue by the experimenter to prevent injury (solid line in Figure 5A). At every risky gap increment, infants were more likely to fall into the gap in their unfamiliar crawling posture compared with their more practiced sitting posture. Six of the 19 infants fell into thin air on every trial in the crawling condition, including the largest 90-cm gap. The remaining 13 infants showed some ability to discern threats to balance control by occasionally avoiding the larger gap distances. However, for this more discerning group of infants, failed attempts were significantly higher in the crawling condition than in the sitting condition at the gap threshold and at +6and + 12-cm increments. Because the data in Figure 5 were based on relative amount of risk, this analysis raises the possibility that posture-specific responding was merely a consequence of the fact that infants' gap boundaries for sitting were larger than for crawling, that is, that infants could lean farther forward in the sitting posture than in the crawling posture. If infants' responses were based simply on absolute gap size, such that they always attempted small gaps and refused large ones, then the finding of posture-specific responding would be spurious. However, examination of the attempt ratio at each absolute gap size shows that this was not the case. Every baby showed different attempt ratios at the same absolute gap size in sitting and crawling postures. The six foolhardy babies who fell into every risky gap in the crawling posture clearly responded differently to the same absolute gap size because they avoided the same gaps in the sitting condition. The subset of 13 more discerning infants also showed different attempt ratios to the same absolute gap sizes in sitting and crawling postures. These infants were actually less likely to attempt gaps between 14 and 32 cm in the crawling posture than the sitting posture. But, as shown in Figure 6, in the crawling posture, their attempt ratios still grossly overestimated their ability to span gaps; attempt ratios were significantly higher than the probability of success, even when the probability of success was zero. In contrast, in the sitting posture, infants' attempt ratios closely matched their ability to span gaps; the curves representing attempt ratios and probability of success were superimposed. Because the experimenter rescued infants after they began to fall, a second alternative explanation for the findings is that infants simply learned to rely on the experimenter to catch them. If infants expected the experimenter to catch them, or if they considered falling into the experimenter's arms to be a kind of game, then babies should have responded indiscriminately to all gap sizes in both postural conditions. However, they did not. Effects were the same regardless of condition order, such that infants were no more or less likely to attempt gaps depending on whether they were rescued in the first condition. Infants were most likely to avoid the largest gaps in the crawling condition, although those gaps appeared latest in the session after infants were rescued multiple times on smaller sizes of gaps. And all infants avoided risky gaps in the sitting condition, despite being rescued in the crawling condition.

16

Karen E. Adolph

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5. Replication We conducted a replication study to rule out a third alternative explanation: Infants' adaptive responding in the sitting posture may have been due to the fact that the landing platform was always 90-cm away. Possibly, view of the large precipice below the toy lure was responsible for their adaptive avoidance responses in that condition. To test this possibility, we tested a new group of infants using the same procedure but now we varied gap distance by moving the landing platform along a calibrated track (0-90 cm). As before, in the sitting condition, a toy was presented on the end of a stick but now the toy was always aligned to the edge of the landing platform, so that by moving the landing platform, we varied the distance of the lure to the baby (see Figure 4C). We increased the baseline gap size to 20 cm in the sitting condition to avoid pinching infants' legs inside the smallest gaps. The experimenter repeated trials on which infants propped their legs or free hand on the far side of the gap to aid in balance control. In the crawling condition, the toy was placed on the landing platform as before.

Learning to Keep Balance

17

Seventeen 9-month-old infants participated. All were more experienced in the sitting posture (M = 3.4 months) than in the crawling posture (M = 1.8 months). With the new experimental set-up, we replicated all results in the first study. In their more experienced sitting posture, infants closely matched attempts to span the gap to the probability of spanning it successfully. But, in their less familiar crawling posture, infants attempted impossibly risky gaps on repeated trials and fell into the precipice (see Figure 5B). In the crawling posture, 8 of the 17 infants fell at every risky gap distance, including the largest 90-cm gap. B. CRAWLING AND WALKING DOWN SLOPES

1. Developmental Design and Rationale The facilitative effects of experience are not limited to maintaining balance in sitting and crawling postures or to avoiding a sheer drop-off at the edge of a gap. In an intense microgenetic study, we found that experience led to specificity of infants' knowledge about balance from crawling to walking postures when they were confronted with steep and shallow slopes (Adolph, 1997). Babies' task was to decide whether to descend the slopes using their typical crawling and walking method, or alternatively to descend in a sliding position or simply avoid going. As in the gaps experiments, we separated the effects of infants' experience and age, this time using a longitudinal design. We tested 15 infants from their very first week of crawling until several weeks after they began walking. Sessions were scheduled 3 weeks apart. An additional group of 14 infants were observed at matched session times (their first and tenth weeks of crawling and first week of walking) to control for the effects of repeated practice on laboratory slopes. Infants in the experimental group were between 5.5 and 9.9 months old at their first crawling session and between 13.6 and 16.1 months old at their 13th-week walking session. Infants in both the control group and the experimental group did not differ in age at the matched test sessions. Overall, infants contributed 219 test sessions. All the parents agreed to prohibit their infants from climbing up or down slopes (sloping yards, wheelchair ramps, playground slides, etc.) outside the laboratory.

2. Slopes Pla(orm and Procedure We constructed an adjustable slope by connecting three large wooden platforms with piano hinges (see Figure 7). The starting platform was always flat and maintained in a stationary position 71-cm high. The landing platform was always flat but could move up and down powered by a hydraulic pump built from a car jack. Moving the landing platform caused the middle section of the walkway to slope in 2 ~ increments from 0~ ~ The entire walkway was covered in soft carpet to provide a cushion and the sides of the walkway were lined with volleyball nets as a safety precaution.

18

Karen E. Adolph

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Because bodies and skills varied widely between infants and sessions, we normalized the definition of safe and risky slopes relative to each infants' current level of crawling or walking skill. We used the same psychophysical procedure described for the gaps experiments to estimate infants' slope thresholds at each week of crawling and walking. The experimenter began with an easy baseline slope of 4 ~ increased the slant by 6 ~ after successes, decreased the slant by 4 ~ after two consecutive unsuccessful trials, and interspersed baseline trials throughout the protocol to maintain infants' motivation. Sessions ended with two trials at the steepest 36 ~ slope. On average, the total number of trials per session was 20.82. Overall infants in the experimental group experienced 102-369 trials descending slopes. Body size and skill level ranged widely between infants and across sessions, pointing to the importance of normalizing the definition of risk to individual infants' current capabilities. On average, from their first crawling session to their 13th walking session, infants' height increased by 12.5 cm, their weight by 2.9 kg, and their head circumference by 4.3 cm. Crawling speed increased by 32.48 crn/s from their first to their last weeks of crawling, and walking step length increased by 10.3 cm from their first to 13th week of walking. Moreover, changes in infants' body dimensions and locomotor skills translated into changing slope thresholds. Slope thresholds increased over weeks of belly crawling, hands and knees crawling, and walking, and decreased at the transitions from one form of locomotion to another. Figure 8 shows changing slope thresholds for a typical infant across weeks of crawling and walking. Thus, a risky slope one week could be perfectly safe the next week after locomotor skill improved. A safe slope for an experienced crawler could be risky for a novice walker, and so on.

Learning to Keep Balance

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3. Flexibility and Specificity in Crawling and Walking Postures As predicted by the sway model, infants displayed both impressive flexibility and striking specificity of knowledge about balance control. To index the accuracy of infants' motor decisions, we calculated each baby's attempt ratio on safe slopes (shallower or equal to the slope threshold) and risky slopes (steeper than the slope threshold). Figure 9 shows infants' average error rates (attempt ratios) on risky slopesmwhen crawlers tried to descend in a crawling position and required rescue by the experimenter and when walkers attempted to descend in a walking position and fell into the experimenter's arms. Because of the nature of the staircase procedure, most of the trials on risky slopes were presented at increments within 8 ~ of the slope threshold. Thus, a low error rate represents an extremely high degree of accuracy.

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0

Control

Fig. 9. Attempt ratios on risky slopes (errors) across weeks of crawling and walking. Infants in the experimental group were tested every 3 weeks. Infants in the control group were tested in their first and tenth weeks of crawling and in their first week of walking. From "Learning in the Development of lnfant Locomotion '" by K. E. Adolph, 1997, Monographs of the Society for Research in Child Development, 62 (3, Serial No. 251), p. 64, Copyright 1997 by the Society for Research in Child Development, Inc. Adapted with permission.

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a. Experience Predicts Adaptive Responding. Figure 9 highlights several important findings. First, the decrease in infants' errors was related to the duration of their crawling and walking experience. In their first weeks of crawling and walking, infants' plunged over the brink of impossibly risky slopes on trial after trial. Over weeks of everyday locomotor experience, errors decreased steadily such that infants used their typical crawling or walking method on safe slopes but slid down or avoided risky ones. Of course, with each week of locomotor experience, infants also became one week older. Several findings point to experience rather than age as the critical determinant of adaptive responding. First, age varied widely at each session (average range = 4.2 months). A maturational component might be expected to show a fighter range in age. Second, infants displayed an increase in errors on risky slopes in their first weeks of walking after 22 or so weeks of crawling, despite their greater age in the walking sessions. And, third, when infants' test age and everyday locomotor experience were pitted directly against each other, experience proved to be the stronger predictor of adaptive responding. Infants' last week of crawling (the week prior to walking onset) provided the strongest test of age versus experience because it was the only session in which both factors varied freely. The duration of infants' crawling experience varied from. 16 to 8.4 months in the experimental group (one child crawled for only a few days prior to walking and others crawled for several months). Age ranged from 9.8 to 14.6 months. Not surprisingly, measures of age and experience were intercorrelated Jr(13) - .61, p < .02]. Older infants tended to have more locomotor experience than younger infants. To compare the unique contributions of age and experience, we conducted partial correlation analyses on attempt ratios on risky slopes. The partial correlation coefficient, r(12), between crawling experience and attempt ratios, controlling for the effects of test age, was -.58, p < .03. In contrast, the partial correlation coefficient, r(12), between test age and attempt ratios, controlling for the effects of crawling experience, was -.33, p -- .25. The fact that errors decreased with weeks of experience eliminates an alternative explanation, that infants simply relied on the experimenter to rescue them. On this alternative account, infants' behaviors might reflect their knowledge about stalwart, friendly adults rather than motor decisions about balance control. The same experimenter caught each infant at each session and most infants experienced dozens of rescues. However, if infants simply learned that the experimenter was going to catch them, they should have become more reckless across test sessions rather than more cautious as was clearly the case. b. Learning Takes a Long Time. A second important finding is that learning takes a really long time. As illustrated in Figure 9, on average, infants required more than 10 weeks of crawling and walking experience before their errors dropped from nearly 75% to below 50%. Most infants required nearly 20 weeks of locomotor experience with each posture before errors became very uncommon (around 10%).

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Moreover, serious falls outside the laboratory had no apparent impact on infants' behavior on the slopes. For example, one infant in the experimental group fell headfirst down a flight of stairs while pushing himself around in a mechanical baby walker 4 days before crawling onset and was rushed to the emergency room with a bloodied and bruised face. At his first test session a few days later, he went headfirst down risky slopes in the laboratory task. Similarly, in analyses of cross-sectional data, behavior on slopes was unaffected by serious falls at home in which infants incurred broken arms/legs, cut lips, or stitches (Adolph, 1995, 1997; Adolph et al., 1993a). Serious falls at home are also apparently unrelated to avoidance responses to a sheer drop-off on the visual cliff (Scarr & Salapatek, 1970). Just as serious falls at home did not facilitate learning in the novel laboratory task, neither did falling down slopes during the laboratory sessions. The strongest evidence for within-session learning over consecutive trials would be if failures on one trial prompted refusals on the very next trial, on the very same slope. However, we found the opposite effect. Although infants found falling to be aversive (they typically fussed after each fall), on 80% of paired trials where their first attempt resuited in falling, they attempted the same maladaptive crawling or walking method on the next trial at the same slope a few moments later. c. Infants Learn to Gauge Balance Control in Real Time. A third important finding illustrated in Figure 9 is that the decrease in errors points to immense psychological flexibility. The naturally occurring changes in infants' bodies and skills cause corresponding changes in the size of their region of permissible postural sway. Nevertheless, infants were able to make adaptive decisions about whether to descend slopes despite weekly changes in their bodies and locomotor skill levels. Additional evidence for flexibility comes from a cross-sectional study with relatively experienced 14-month-old walking infants (Adolph & Avolio, 2000). We experimentally manipulated the size of infants' sway region by loading them with shoulderpacks that varied in weight from trial to trial (120 g and 25% of infants' body weight). With the heavy shoulderpacks, the region of permissible postural sway was reduced by 30% and infants' slope thresholds showed a corresponding reduction of 4~ ~. Despite constant switching between trials of their shoulderpacks, infants could accurately gauge threats to balance as they walked down steep and shallow slopes. They treated the same absolute degree of slope as safe while wearing their light shoulderpacks but as risky while wearing their heavy shoulderpacks. On the sway model, both naturally occurring and experimentally induced changes in the sway region would require continual updating about infants' locomotor abilities relative to the degree of slant. Coping with moment-to-moment changes in the size of the sway region means that infants must learn to gauge threats to balance control in real time, in the context of their current abilities, goals, and the particular constraints of the task.

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A related point about flexibility concerns the breadth of infants' generalization. Apparently, learning resulted from uneventful everyday experience maintaining balance in crawling and walking postures at home, not from experience descending slopes. None of the infants had experiences on slopes outside the laboratory. Moreover, the control infants (represented by the open symbols in Figure 9) who had no weekly experience descending laboratory slopes showed nearly identical decisions after comparable experience crawling and walking as the babies in the experimental group. A final point about flexibility concerns the consistency of infants' responses within sessions. At each test session, infants' attempt ratios were scaled to their slope thresholds. They were most likely to attempt risky slopes closest to their thresholds and least likely to attempt risky slopes most remote from their thresholds. In 89% of the 219 individual session protocols, infants' responses were entirely consistent on risky slopes, where attempt ratios were constant or steadily decreasing from the threshold slope to the steepest increment. Over weeks of crawling and walking experience, infants' attempt ratios decreased on slopes closer to their thresholds, until, finally, attempt ratios closely matched the conditional probability of success. Thus, learning reflected a process of"gearing in" to the limits of infants' abilities, a process of gradually differentiating the slopes which presented threats to balance from those that did not.

d. No Transfer from Crawling to Walking. The most striking result shown in Figure 9 was the specificity of infants' knowledge across developmental changes in posture. Learning curves across weeks of crawling and walking were separate and knowledge gained from crawling did not transfer to walking. The same infants who had responded adaptively for weeks in their old, familiar crawling posture, attempted to walk straight over impossibly steep slopes in their new, unfamiliar walking posture. Errors were just as high in the first weeks of walking as they were in the first weeks of crawling and learning was no faster the second time around. As a final test of the specificity of infants' knowledge about balance control, we tested infants in their first weeks of walking with six back-to-back trials at the steepest available 36 ~ slope: These included two trials at 36 ~ in their new uptight walking posture, then two trials in their old, familiar crawling posture, then two trials again in their new uptight posture. As in the gaps studies, this manipulation provided a way to control for infants' age while comparing their knowledge in more versus less experienced posture. Although both hands-and-knees crawling and walking were impossible at the steepest 36 ~ slope, infants' knowledge was posture specific. When new walkers faced the slope in their old crawling position, their decisions were extremely accurate, but when they faced the slope in their new uptight position, their decisions were fraught with errors. On average, the rate of falling when starting on hands and knees was .05, equally accurate as the rate of falling in the week prior to walking onset (M = .07 after 20 weeks of crawling). However, when new walkers began

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trials in an upright posture, their rate of falling was .30. Even after a reminder that the slope was risky in the crawling position and that several sliding strategies were available in their repertoires, babies continued to walk over the edge and fall in their final two walking trials. Moreover, many new walkers insisted on facing the obstacle in their new uptight posture, as though preferring to face the slope as hapless walkers rather than experienced crawlers. Despite being placed by the experimenter on their hands and knees, 10 infants in the experimental group and 7 infants in the control group stood themselves up and walked blithely over the edge and fell. Errors after standing themselves up were equally high (M = .38) as when infants began the trials in an upright posture (M = .30). Both the specificity and the flexibility of infants' behaviors argues against an alternative explanation that infants' responses were based on the absolute degree of slant rather than on the relative amount of risk. Because degree of risk has an ordinal nature--riskier slopes are steeper than safer slopes--infants might simply associate aversive consequences with the steepest slopes or learn to associate sliding and avoidance with the steepest slopes. That is, if babies simply learned to associate particular responses with absolute degree of slope, then they should have learned to attempt shallow hills and slide down steep ones regardless of changes in their level of locomotor skill and developmental changes in posture. However, they did not. Novice crawlers and walkers treated the steepest slopes as safe, regardless of their skill level: They plunged down 36 ~ and fell. Experienced crawlers and walkers geared their responses on the steepest slopes precisely to their skill level. Less proficient infants slid down or avoided going, more proficient infants used their typical crawling or walking method, and falls among experienced infants were exceedingly rare. 4. Summary

The slopes and gaps studies provide strong support for the sway model. In short, infants avoid falling by maintaining their bodies within a region of permissible postural sway. Because the size of the sway region changes from moment to moment, infants must learn to gauge their region of permissible postural sway in real time. Developmental changes in posture create new balance control systems. Each time infants acquire a new postural milestone, they must identify the relevant control parameters which define the new sway region and learn to calibrate its settings. As a result, adaptive responding in novel predicaments should increase with everyday experience in a given posture, and such responding should not transfer across developmental changes in posture. c. LEARNINGTO DETECTTHREATSTO BALANCE:THE VISUAL CLIFFAND OTHERFALLINGTASKS The sway model can account for developmental changes in infants' successes and failures in a wide range of motor tasks reported in the literature. The common

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feature across tasks is that babies must avoid falling. Although most tasks were not conceived by their original designers in terms of balance control, I argue that the critical factor underlying infants' performance is their ability to gauge their region of permissible postural sway. 1. The Visual Cliff and Locomotor Experience

The best known "falling" task, of course, is the "visual cliff" (Gibson & Walk, 1960; Walk & Gibson, 1961). Since Gibson and Walk's classic studies, several researchers have found that human infants and other altricial animals require a protracted period of locomotor experience before they avoid crawling over the deep side of the visual cliff (Campos, Bertenthal, & Kermoian, 1992; Held & Hein, 1963; Rader, Bausano, & Richards, 1980; Richards & Rader, 1983; Walk, 1966; Walk & Gibson, 1961). In a particularly clever age-matched control design, Bertenthal, Campos, and Barrett (1984) showed that the duration of infants' everyday crawling experience predicts adaptive avoidance responses, independent of infants' age at testing or the age at which they began crawling. At the very same age at testing (7.5-8.5 months) only 35% of inexperienced crawlers (M = 11 days of crawling experience) avoided the apparent drop-off but 65% of more experienced infants (M -- 41 days) refused to crawl over the precipice. Several accounts have been proposed about what infants may learn during this period of experience that facilitates the coordination between perception and action. The early pioneers, Gibson and Walk (Gibson & Walk, 1960), proposed that infants must acquire depth perception to avoid the drop-off on the visual cliff. We now know that avoiding a cliff does not depend solely on depth perception because even newborns can differentiate disparities in depth (Campos, Langer, & Krowitz, 1970; Slater and Morison, 1985) and precrawling infants can use depth information adaptively to guide their movements in reaching tasks (Gordon & Yonas, 1976; McKenzie, Skouteris, Day, Hartman, & Yonas, 1993; Yonas, Granrud, Arterberry, & Hanson, 1986; Yonas & Hartman, 1993). The most widely cited modem explanation is that everyday crawling experience leads to fear of heights. This emotional response to depth information mediates adaptive avoidance responses (Campos et al., 1978, 1992). A different sort of explanation relies on associative learning about various kinds of surfaces and their consequences for locomotion. Infants may learn that cliffs are dangerous, for example, from experiences peering over the edges of sheer drop-offs and from many near-falls as vigilant parents grab them at the edge of the bed or the changing table (Thelen & Smith, 1994). Finally, others have proposed that experience leads to an appreciation of the properties of the ground surface for supporting the body (Bertenthal & Campos, 1990; Gibson & Schmuckler, 1989). In particular, experience crawling over solid ground might teach infants that locomotion is impossible without a substantial surface that they can see and feel beneath their hands and feet.

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According to the sway model, none of these accounts is sufficient for explaining how experience facilitates adaptive responses to depth information for a drop-off. If infants learn to avoid a drop-off because they are afraid of heights, know that cliffs are dangerous, or know that their bodies cannot be supported in empty space, then they should show similar adaptive avoidance responses to a drop-off regardless of the posture in which they are tested. However, our observations of infants at the edge of a sheer drop-off on gaps and at the brink of a graded drop-off on steep slopes showed that infants' learning is specific to each postural milestone in development. 2. Other Falling Tasks Although the visual cliff is the most widely known test paradigm, for more than a half century, researchers have devised novel and innovative tasks in which babies must avoid falling. Some tasks were designed to test biomechanical models of balance control by measuring babies' responses to unexpected disruptions of their balance. For example, infants were tested in sitting and standing positions on movable floors which jerked suddenly forward or backward, similar to the lurch of a subway car (Shumway-Cooke & Woollacott, 1985; Woollacott & Sveistrup, 1992; Woollacott, Hofsten, & Rosblad, 1988). Some tasks were designed to demonstrate the importance of vision or touch as a source of proprioceptive information about where the body is in space. Infants were observed trying to sit, stand, or walk in rooms where the walls jerked forward and backward to simulate the optical information generated by spontaneous forward and backward body sway (Bertenthal, Rose, & Bai, 1997; Lee & Aronson, 1974; Schmuckler & Gibson, 1989; Stoffregen et al., 1987). They were tested standing on special force platforms with their hands resting on a supporting bar to determine whether their body sway attenuates when they have something to grip onto (Metc~lfe & Clark, 2000). Other tasks were designed to test the proposal (Bernstein, 1996; Reed, 1982) that balance control underlies all perceptual-motor behaviors. Researchers measured babies' control of balance in the context of suprapostural goals such as reaching for objects at various distances and locations while keeping balance in a sitting position (McKenzie et al., 1993; Rochat, 1992; Rochat & Goubet, 1995; Rochat & Senders, 1991). Their postural responses were measured with electromyography as they engaged in reaching movements (Hofsten, 1993; Woollacott et al., 1988). More far afield, researchers designed tasks to test infants' perception of affordances for locomotion (Adolph, Eppler, & Gibson, 1993b; Gibson, Adolph, & Eppler, 1999; Gibson, 1979), to test their ability to manipulate environmental supports as tools to aid their locomotion, and to catalog developmental changes in common skills such as stair-climbing. Babies were observed coping with stance and locomotion on slippery and sticky surfaces (Lo, Avolio, Massop, & Adolph, 1999; Stoffregen, Adolph, Thelen, Gorday, & Sheng, 1997) and on squishy and rigid ones (Gibson et al., 1987). They were challenged to step over or under barriers

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Karen E. Adolph

(Schmuckler, 1996; Van der Meer, 1997), to squeeze through wide and narrow apertures (Palmer, 1987), and to detour around obstacles in their path (Gibson & Schmuckler, 1989; Lockrnan, 1984; Schmuckler & Gibson, 1989). They were encouraged to cross wide and narrow bridges with and without handrails to hang onto (Berger & Adolph, 2002a, 2002b). They were asked to climb up and down stairs (Gesell & Thompson, 1938; Ulrich, Thelen, & Niles, 1990), ladders (McCaskill & Wellman, 1938), tall pedestals (McGraw, 1935), and steep slopes (Adolph, 1995, 1997; Adolph et al., 1993a; Eppler, Adolph, & Weiner, 1996; McGraw, 1935). They were even tested walking down slopes with weights attached to their shoulders (Adolph & Avolio, 2000) and filmed descending slopes while roller skating backward (McGraw, 1935)! The sway model provides a unifying psychological framework for understanding developmental changes in infants' performance across the range of falling tasks. Regardless of whether the tasks were originally designed to measure balance control, reaching, fear of heights, perception of surface affordances, tool use, or any other construct, the falling tasks share the common requirement of gauging in real time the current size of the sway region. That is, infants' success in each task depends on their ability to detect and respond appropriately to potential threats to balance. Moreover, every task shows a similar developmental progression. As in the visual cliff, gaps, and slopes studies, the adaptiveness of infants' responses depends on the duration of their everyday experience and their age at testing (Adolph, 1997). Older, more experienced infants show more mature muscle responses to a jerking floor, maintain balance more effectively in a moving room, are more likely to avoid crawling or walking down impossibly steep slopes, wend their way more successfully around barriers in their path, and so on. On the sway model, everyday experience should facilitate adaptive responding and experience should be specific to each posture in development. Unfortunately, few investigators have tried to understand the separate effects of age and experience in this myriad of tasks or have tested infants across developmental changes in posture. However, the available data suggest that experience is the stronger predictor of adaptive responding and that learning does not transfer across developmental changes in posture. As on the visual cliff, gaps, and slopes, experience is a better predictor than age or walking skill of infants' responses when confronted by high and low barriers in their path (Kingsnorth & Schmuckler, 2000; Schmuckler, 1996). More experienced infants are better able to judge safe heights for stepping over the barriers. As in the gaps and slopes studies, experienced crawlers avoided crossing the visual cliff when facing it from their familiar crawling position. However, the same infants went straight over it when supported uptight in mechanical baby walkers (Rader et al., 1980). Similarly, infants' ability to execute reaching detours around barriers increased over weeks of sitting, but the same infants had to relearn how to execute detours to negotiate barriers when

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they were tested while in a crawling posture (Lockman, 1984). Finally, infants' ability to maintain balance in a moving room or on a jerking floor apparently does not transfer from sitting to standing postures (Bertenthal et al., 1997). In sum, the sway model provides a unifying framework for understanding how infants come to respond adaptively in a wide range of tasks in which the penalty for error is falling down. Common to each task is behavioral flexibility--the ability to generate new solutions to new challenges to balance control on a momentto-moment basis. Consistent with the predictions of the sway model, extended periods of learning appear to occur during everyday experience maintaining balance in each posture, and learning does not transfer across developmental changes in posture.

IV. How Development May Constrain Motor Learning A. CONTENT OF EVERYDAY EXPERIENCE

In the previous sections, I described how adaptive responding in novel laboratory tasks depends on the duration of infants' everyday locomotor experience. Moreover, the results of several studies showed that extended periods of experience---on the order of several weeks or months--are required before infants display consistently adaptive responses in novel laboratory tasks that challenge balance control. In this section, I ask how infants might learn about balance control in the course of everyday experience. That is, what sorts of everyday experiences could promote learning that is flexible enough to cope with novel challenges to balance but specific enough to ensure that infants identify the relevant parameters of each new balance control system in development? Although many researchers have engaged in theorizing about learning mechanisms related to infants' locomotor experiences (e.g., Acredolo, 1988; Adolph, Vereijken, et al., 1998; Bertenthal, Campos, & Kerrnoian, 1994; Gibson, 1988; Piaget, 1954; Richards & Rader, 1983; Thelen & Smith, 1994; Walk, 1966), ironically, the theorizing to date has been largely unconstrained by empirical facts about infants' everyday locomotor experiences. In my own work, such as the gaps and slopes studies described earlier, in the work of the other investigators who observed infants in "falling" tasks, and elsewhere in the literature where investigators posited a role of locomotor experience in infants' development, experience has been treated similar to age: Experience is simply the number of days elapsed between the onset of a skill and the test date. There are a number of serious problems inherent in quantifying locomotor experience in terms of elapsed time. The most serious problem is that elapsed time is not an explanatory variable. Just as age differences alone cannot explain developmental progress, experience differences cannot address underlying learning

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mechanisms without a description of the critical experience-related changes that occurred during the elapsed time between onset and test dates. The only purported mechanism that has been examined empirically is one-trial learning from serious falls, that is, some sort of fast-mapping between perceptual information for disequilibrium and the aversive consequences of falling (Bertenthal et al., 1984). On this account, overall duration of locomotor experience may predict adaptive responding because the longer the children travel around independently, the more likely they are to have experienced a serious fall. An analogy might be that long-time drivers respond more adaptively than student drivers to novel challenges because the former have had more opportunities to learn from memorable and aversive events such as steering out of a skid or avoiding a rear-end collision. As described previously, the existing data do not support such a mechanism. Very few parents report that their infants have incurred serious falls during everyday locomotion and those infants that do fall are no more likely to exhibit adaptive responses in novel "falling" tasks in the laboratory than infants who have not experienced serious mishaps (Adolph, 1995, 1997; Scarr & Salapatek, 1970). With the exception of parents' reports about falling accidents, to my knowledge, there are no published data that describe the actual content of human infants' everyday motor experience--how frequently infants locomote, where they go and how often, how far they travel, and what surfaces and paths they traverse. A second problem with quantifying locomotor experience in terms of elapsed time concerns the way that duration of experience is calculated. To determine an onset day, the researcher must set a definitional criterion (crawls a distance of 90 cm on hands and knees, walks a distance of 305 cm without holding onto parents or furniture for support, etc.). The problem is that motor skills typically appear in a graded fashion, where earlier appearing behaviors are previews of the later appearing target skills (e.g., Gesell, 1946; McGraw, 1945). Thus, infants may acquire a significant amount of relevant experience prior to passing the criterion for the onset day. For example, infants may crawl or walk slightly less than the criterial distances or exhibit precursory approximations to the final skill (e.g., crawl on belly, pivot in circles, balance on hands and knees, and so on, prior to crawling to criterion on hands and knees). Such smaller bits of experience with earlier appearing approximations to a target skill have proven to be powerful predictors of performance with the target skill weeks later (Adolph, Vereijken, et al., 1998). A third problem with quantifying locomotor experience in terms of elapsed time concerns the developmental trajectory of motor skill acquisition across days. By equating duration of experience with the number of days between onset and test days, researchers inherently assume that developmental trajectories are abrupt rather than gradual or variable. That is, infants who pass criterion for a target skill on Monday should perform the skill on Tuesday and Wednesday and each successive day between onset and test. However, if developmental trajectories do not always turn on and off like a faucet so that each day is like the last, then experience is not

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necessarily meted out in daily doses. That is, an infant who passes criterion for a skill on Monday may not demonstrate the skill again until Friday or may pass criterion only every other day, and so on. Finally, the duration and content of infants' experience is typically estimated from parents' retrospective reports. With the exception of a few longitudinal studies where infants were tracked prospectively (Adolph, 1997; Bertenthal et al., 1984; Eppler et al., 1997; McGraw, 1935), parents must rely on their memories and incidental records to report onsent dates and special experiences such as serious falls, exposure to stairs and slides, and so on. Our data suggest that with careful probing, structured interview methods, and support from parents' "baby books" and calendars, parents' reports of onset dates are relatively accurate; for example, duration of experience calculated from retrospective reports is well correlated with objective measures of locomotor skill such as step length and step velocity. However, retrospective reports are subject to smoothing errors and cannot be independently verified. It is difficult for parents to remember which day their infants crawled 90 cm or walked 305 cm during everyday locomotion at home when they are queried weeks or months later in the laboratory. To redress the problems in understanding experience in terms of elapsed time, we have designed several convergent prospective diary methods to describe what infants' everyday locomotor experience really entails. Because the diaries are prospective rather than retrospective we hope to minimize errors in parents' reports. Our aim is to compile a rich, archival database of everyday experience in a sample of healthy, typically developing infants and to thereby set boundary conditions on possible learning mechanisms.

1. Daily Frequency of Infant Locomotion We devised a "checklist diary" method to test the daily frequency of infants' locomotor experience (Adolph, Biu, Pethkongathan, & Young, 2002). We train parents to check off 56 motor skills, events, and instances of passive locomotion on a daily basis. Each motor skill is defined according to a strict, standard criterion. For example, forms of locomotion include belly crawling, hands-and-knees crawling, walking, and running a minimum of 305 cm (10 feet) consecutively, and cruising (walking sideways holding onto furniture for support) three consecutive steps (the length of a standard couch). Precursors to bonafide locomotion include taking one or two forward steps on belly, hands and knees, or feet; pivoting in circles on belly; and so on. The events include serious falls (resulting in bruises, bumps, breaks, or blood); serious illnesses or family trips that may have prevented infants from engaging in normal gross motor activities (bed-bound, traveling in car for long stretches of the day, etc.); and classes that may have presented infants with special motor experiences (e.g., Gymboree). To date, five infants in the New York City area have completed participation in the checklist diary study. Parents began the diaries when their infants were 1.2-4.0

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months old (all prior to rolling or sitting independently) and stopped keeping the diaries when their infants were 14.6-19.2 months old. During monthly visits to the laboratory, we tested infants on the list of skills, queried parents' about their diary entries, and refreshed parents' criteria for noting each skill. Preliminary data suggest that the practice of quantifying locomotor experience in terms of days between onset and test days may indeed promote an erroneous understanding of infants' everyday experience. In every case where infants demonstrated a target locomotor skill, they displayed precursory approximations to the skill prior to its onset date (and, in the case of crawling, they sometimes demonstrated precursors without ever passing criterion for the target skill). For example, they managed to maintain static postures or to take a couple of faltering steps prior to traveling the criterion distances. Both precursory skills and bonafide locomotion showed abrupt (1 day) transitions in only 18 of 70 possible cases. In all of the other cases, skills appeared gradually such that infants demonstrated the skill for a day or two but failed to demonstrate it on succeeding days, sometimes stretching the on-off pattern over several weeks. Similarly, crawling and cruising disappeared gradually (for five of eight possible cases) such that infants stopped performing the skill for a day or two but sometimes performed it on succeeding days. Across skills, transitional periods ranged from 1 to 107 days. Even the periods of presumed stability that flank transitional periods were marred by occasional appearances or disappearances of the target skill. Thus, the developmental trajectory of infants' motor skills is not necessarily stage like, "turning on" or "off" like a faucet from one day to the next; the frequency is not necessarily distributed regularly across days, and more days between onset and test days does not guarantee more days of experience. Rather, the daily frequency of locomotor skills is typically messy and uneven, both in the transitional periods when skills first appear in or disappear from infants' repertoires and during the flanking periods when skills are relatively stable in their presence or absence. In addition, in accordance with previous research, we found that infants' locomotor experience was generally uneventful. Only one infant (my own!) experienced a serious fall which required medical attention. Neither the serious fall nor the everyday bumps that all the infants experienced appeared to hinder their progress at locomotion. The only circumstance that kept infants from locomoting was preventing their access to the floor. Several families went on vacations during which infants were cooped up on long car tides, penned in a tiny camping tent or play space, or otherwise denied access to the floor or to large enough floor spaces to pass criterion for locomotion. One child had a serious medical illness which kept him bed-bound for several weeks.

2. The Nature of lnfants' Travels Our "telephone diary" method is designed to obtain detailed information about the content of infants' crawling experience on a minute-to-minute basis (Chan et al.,

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1999, 2000). With the telephone diary method, we train parents over a 4- to 6week period to track their crawling infants' active and passive locomotion from the time babies wake up until the time they go to bed. Parents telephone throughout the day to a time-stamped answering machine about where their babies went and how they got there. A researcher calls the parents at the end of each day to verify the information and to fill in missing gaps in the protocols. Then, from detailed blueprints of the infants' homes, we reconstruct their crawling paths, the distance that they traveled, the locations that they visited, and so on. To date, seven infants have participated in versions of the telephone crawling diary. My own daughter began testing as a prelocomotor infant at 2 months and completed her last diary entry 129 days after she began crawling to criterion (10 feet consecutively). Three infants began testing on their first day of crawling between 6.5 and 12.3 months of age and completed testing 13.7-16.4 weeks after crawling onset. Three infants began testing 53-100 days after crawling onset (between 10 and 10.5 months of age) and completed testing 3.1-4.1 weeks later. Our preliminary data show that prior to crawling onset, passive locomotion occupied only 7.3% of the day, on average, and time on the floor occupied only 15.9% of the day. However, after crawling onset, infants acquired massive amounts of varied experiences with independent locomotion and balance control. They spent, on average, 16.8% of their walking day in passive locomotion (being carried or wheeledin strollers) and an additional 40.8% of their day on the floor (crawling, cruising, and playing). In terms of raw numbers, infants spent 5.1 hr per day, on average, in active balance and locomotion and 2.6 hr per day in passive locomotion. Although infants spent most of their floor time in the common family spaces (kitchen/family/eating room areas), each day they managed to travel through most of the open floor spaces in most of the rooms of their homes, visiting 5.7-12.3 different surfaces, on average, en route. On average, infants crawled 27.1-42.6 m/hr and covered a total daily distance of 60.4-187.8 m. Given that crawlers average 17 steps per meter while crawling continuously over the laboratory floor, the preliminary telephone diary data suggest that infants experienced 1028-3198 crawling steps per day. Nearly all of infants' forays (97.2%) were one-way trips punctuated by playing or stopping to rest.

3. Quantifying Steps Our "step counter diary" method is designed to quantify infants' walking steps directly and automatically (Adolph, Avolio, Barrett, Mathur, & Murray, 1998). We outfit walking infants with tiny foot switches in their shoes. A Gaitrite paper-thin pressure-sensitive pad is slipped into the insole of their shoes and attached to a tiny micromemory chip and battery clipped to the outside of their shoes. Parents keep a daily checklist diary of the rooms and floor coverings infants travel over and of visits to outside surfaces. At the end of the testing period, we download the data into a computer and calculate the number of walking steps per time block. Preliminary

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data with three 14-month-old toddlers using a beta-test version of the step counter showed that infants were on the floor more than 50% of a typical 12-hr walking day and crossed most of the floor coverings in most of the rooms in their homes. Each infant registered between 500 and 1500 steps per hour. The normal cadence for a 14-month-old toddler is, on average, 190 steps per minute while walking continuously over the laboratroy floor, and each step length averages 30 cm. Thus, the preliminary step counter data indicate that infants' walking experience occurs in fits and starts, with bursts of activity separated by rest periods where they stand still or play. By the end of the day, they may have traveled a total distance of 2700 m.

4. How Experience Promotes Learning In the course of everyday experience with each posture in development, infants execute various exploratory movements, glean the resulting perceptual information, practice compensatory sway responses, and acquire a repertoire of alternative strategies when maintaining balance is impossible. How, then, does learning work? The primary aim of the diary studies was to describe the actual content of everyday locomotor experience in human infants. The hope was that a rich set of descriptive data, obtained with convergent methods, might constrain theorizing about how experience promotes adaptive motor control. Despite the preliminary nature of the data, the small sample sizes, and large individual differences, four findings seem clear and important. First, in accordance with previous studies, our diary data showed that infants' locomotor experience is generally happy and uneventful. Infants rarely experience serious falls or incur frightening episodes with locomotion or balance. Second, infants' experience occurs in massive daily doses compounded over the days prior to and following their official onset days. Third, infants gain experience with stationary balance and locomotion under wildly variable conditions in terms of surfaces, places, and events. And fourth, during the course of each day, experience with locomotion is always interspersed with frequent rest periods. From day to day, experience with locomotion typically alternates between practice days and rest days, especially during the acquisition period. How might these findings help us to understand possible learning mechanisms? The finding that experience is generally positive speaks against one-trial learning. Indeed, the data suggest the opposite: Infants' learning about balance control may occur over many thousands of trials during the course of each day. Even the most elaborate laboratory training studies cannot begin to approximate the massive amount of experience that infants obtain in everyday locomotion. To put the sheer magnitude of infants' locomotor experience into perspective, each day crawling infants practice keeping balance for more than 5 hr; during that time, they can travel the lengths of two football fields and take more than 3000 crawling steps. Walking infants practice keeping balance for more than 6 hr per day; during that time, they can travel the lengths of 29 football fields and take 9000 walking steps. In addition to the time that they are in transit, crawlers, cruisers, and walkers

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gain hours of daily experience maintaining balance in stationary positions. Each little crawling and walking step, each forward lean and reach, each time infants roll over or sit up is a tiny "trial" with balance control. These daily experiences are compounded over the total protracted period of experience, beginning with infants' partial approximations of a new postural milestone, stuttering through the transition period when they first meet criterion for the skill proper, and continuing through the period when the skill is relatively stable in their repertoires. Our preliminary data suggest that infants' locomotor experience should be tightly measured in epochs. Similar to the immense amounts of daily practice which promote expert performance in musicians, athletes, typists, and chess players (Bryan & Harter, 1897, 1899; Ericsson & Charness, 1994; Ericsson, Krampe, & Tesch-Romer, 1993), there are hundreds of thousands of trials spread over several weeks or months before babies show adaptive responses in novel laboratory tasks which challenge balance control. Infants' massive accumulated practice is not synonymous with dull, rote, drudgery. Babies are not performing the same movements over and over in response to the same goals and impediments in the same environmental context, as if on a "blocked" practice regimen in the laboratory where participants are presented with the same problem on successive trials. Rather, infants' cope with balance control in an astounding variety of events, places, and surfaces--even in the familiar setting of their homes and play yards. The changeability and unpredictability of everyday experience resembles an exaggerated version of "variable" or "random" practice, where environmental conditions such as stimulus increment and trial order vary from one attempt to the next. In the laboratory, variable and random practice are detrimental for performance during acquisition of a skill but lead to better performance during transfer tasks under novel conditions. Conversely, blocked practice is beneficial for performance during acquisition but leads to decrements during transfer (Gentile, 2000; Schmidt, 1988). A classic explanation for the difference between blocked and variable/random practice is that the former leads to repeating a particular solution over and over on successive trials whereas the latter leads to a process of continually generating new solutions or generating old solutions anew (Gentile, 2000; Schmidt, 1988). The change in context during variable/random practice may actually prevent infants from merely repeating solutions by causing interference so that the current solution exits working memory and infants are required to construct or reconstruct a solution on successive attempts. Similarly, changing the context from moment to moment may dissuade infants from relying on simple associations between stimuli and responses and force them instead to track the complex constellation of relevant factors for calibrating their current region of sway. Put simply, variety of experience promotes "learning to learn," rather than learning particular solutions (Bernstein, 1967, 1996; Harlow, 1949, 1959). Such variety of experience might be exactly the kind of practice that infants need to achieve adaptive responding when challenged with novel threats to balance.

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The rest periods between bouts of locomotion during each day and the rest days between practice days when skills are displayed may function to increase contextual interference. That is, by stopping to play or socialize between bursts of locomotion, infants change the motivational and biomechanical context of their activity. During the next crawling foray or the next stationary posture, infants must solve the problem of balance control all over again. Alternatively, as in laboratory studies of distributed and massed practice, infants' intermittent experiences with locomotion within and across days may provide them with time to consolidate learning or to allow fatigue or flagging motivation to dissipate (Schmidt, 1988). In sum, the descriptive diary data suggest that infants' everyday opportunities for learning involve the sort of training regimen that promotes expert performance, adaptability, and transfer in laboratory studies of skill acquisition: large amounts of variable, distributed practice. B. ENSURING FLEXIBILITY AND SPECIFICITY

The final section of this chapter addresses a long-standing problem in developmental psychology, understanding the relation between learning and development. The problem of balance control provides a particularly apt illustration of how peripheral developmental changes can constrain the course of learning. By peripheral developmental changes, I refer to ongoing changes that are not linked to the target behavior via a neural prescription in the central nervous system (Thelen & Smith, 1994). As conceived within the context of the sway model, peripheral developmental changes play a central role in ensuring that learning is sufficiently flexible to cope with a variable world and sufficiently specific that infants learn what they need to know. On the sway model, sitting, crawling, and walking are different balance control systems: A hip-torso system, a prone system, and an uptight system. Infants must learn to identify the relevant control parameters that are specific to each of these systems--the relevant muscle groups to control sway, the relevant sensory input to anticipate disruptions of balance, the relevant effectors to generate perceptual information, and so on. How might peripheral developmental changes ensure the necessary specificity to enable infants to identify the relevant balance control parameters? As infants acquire the ability to assume new postures in development, they are forced to search out the new defining parameters because the old parameters simply do not work. If they rely on their old balance control system to guide them, babies will fall over the cliff, or into the gap, or down the slope. The creation of new balance control systems forces infants to learn about the new control parameters. On the sway model, maintaining balance in any posture requires infants to continually monitor the current size of the sway region. Infants must learn to recalibrate their movements, with infinitely sensitive adjustments for each potential

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disruption of balance. Rather than learning facts or rules ("cliffs are dangerous," "descend stairs by crawling backward," "I'm a poor crawler, . . . . my body is fat/thin/tall/short"), infants must learn how to gauge threats to balance in real time. Similar to Harlow's (Harlow, 1949, 1959) notion of "learning to l e a r n , " flexibility in motor control requires much more than simple associative pairing. How might peripheral developmental changes ensure the necessary flexibility to allow infants to cope with unlimited, continual variability? One avenue for constraining the course of learning is by maintaining variability in infants' bodies, skill levels, and environments. In a sense, infants are prevented from learning particular facts or rules for keeping balance because as soon as a fact would be consolidated, the circumstances have again changed: Babies' bodies have grown, their skill level has improved, and they have been exposed to a new array of surfaces and events. A second avenue in which peripheral developmental changes might constrain the course of learning is by extending the learning period. Individual infants' experiences differ widely in terms of opportunities for learning. Chubbier, less muscular babies must work harder to locomote through the environment. Less skillful infants gain access to a subset of the locations and events available to more skillful infants. Infants in small New York City apartments have smaller arenas to travel in compared with infants in large suburban homes. Infants in cold climates are bundled into more clothing and have less frequent access to outdoor surfaces. Nonetheless, all healthy babies eventually learn to keep balance and respond adaptively to novel challenges. On the sway model, the specific experiences do not matter but variable practice across different local conditions does. By extending the learning period so that infants spend weeks or months in each posture, infants are ensured distributed practice keeping balance over time, surfaces, and events. C. CONCLUSION

In this chapter, I have presented the sway model in an effort to provide a unifying framework for understanding infants' performance on the visual cliff and dozens of other tasks in the literature on perceptual-motor development. In addition, I had loftier aims. I have argued that keeping balance is not merely a biomechanical problem for movement scientists and sports enthusiasts. It is a manifestly psychological problem that is central to understanding motor control. I have also argued that studies of infants falling can speak to the general problem in developmental psychology of the relation between learning and development. Balance control provides a rich example of how peripheral developmental changes can constrain learning without building knowledge anywhere into the system. Finally, balance control highlights the importance of psychological flexibility, one of the most wondrous achievements of all.

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ACKNOWLEDGMENTS This research was supported by NICHD Grant #HD33486 to Karen Adolph. I gratefully acknowledge Anthony Avolio, Marion Eppler, Ann Gentile, Eleanor Gibson, and Esther Thelen for their help in understanding the data; members of my Infant Motor Development Laboratory at Carnegie Mellon University and New York University for their help in data collection, coding, and analyses; and the members of Esther Thelen's Infant Motor Development Laboratory at Indiana University for their help in data collection.

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Gesell, A., & Thompson, H. (1938). The psychology of early growth including norms of infant behavior and a method of genetic analysis. New York: Macmillan. Gibson, E. J. (1988). Exploratory behavior in the development of perceiving, acting and the acquiring of knowledge. Annual Review of Psychology, 39, 1-41. Gibson, E. J., Adolph, K. E., & Eppler, M. A. (1999). Affordance. In E Keil (Ed.), Encyclopedia of the cognitive sciences. Cambridge, MA: MIT Press. Gibson, E. J., & Pick, A. D. (2000). The ecological approach to perceptual learning and development. New York: Oxford University Press. Gibson, E. J., Riccio, G., Schmuckler, M. A., Stoffregen, T. A., Rosenberg, D., & Taormina, J. (1987). Detection of the traversability of surfaces by crawling and walking infants. Journal of Experimental Psychology: Human Perception and Performance, 13, 533-544. Gibson, E. J., & Schmuckler, M. A. (1989). Going somewhere: An ecological and experimental approach to development of mobility. Ecological Psychology, 1, 3-25. Gibson, E. J., & Walk, R. D. (1960). The "visual cliff." Scientific American, 202, 64-71. Gibson, J. J. (1958). Visually controlled locomotion and visual orientation in animals. British Journal of Psychology, 49, 182-194. Gibson, J. J. (1979). The ecological approach to visual perception. Boston: Houghton Mifflin. Goldfield, E. C. (1989). Transition from rocking to crawling: Postural constraints in infant movement. Developmental Psychology, 25, 913-919. Gordon, E R., & Yonas, A. (1976). Sensitivity to binocular depth information in infants. Journal of Experimental Child Psychology, 22, 413-422. Harlow, H. E (1949). The formation of learning sets. Psychological Review, 56, 26-39. Harlow, H. E (1959). Learning set and error factor theory. In S. Koch (Ed.), Psychology: A study of a science (pp. 492-533). New York: McGraw-Hill. Held, R., & Hein, A. (1963). Movement-produced stimulation in the development of visually guided behavior. Journal of Comparative and Physiological Psychology, 56, 872-876. Hofsten, C. (1993). Prospective control: A basic aspect of action development. Human Development, 36, 253-270. Kingsnorth, S., & Schmuckler, M. S. (2000). Walking skill versus walking experience as a predictor of barrier crossing in toddlers. Infant Behavior and Development, 23, 331-350. Lampl, M. (1993). Evidence of saltatory growth in infancy. American Journal of Human Biology, 5, 641-652. Lampl, M., Veldhuis, J. D., & Johnson, M. L. (1992). Saltation and statis: A model of human growth. Science, 258, 801-803. Lee, D. N., & Aronson, E. (1974). Visual proprioceptive control of standing in human infants. Perception and Psychophysics, 15, 529-532. Lee, D. N., & Lishman, J. R. (1975). Visual proprioceptive control of stance. Journal of Human Movement Studies, 1, 87-95. Leo, A. J., Chiu, J., & Adolph, K. E. (2000, July). Temporal and functional relationships of crawling, cruising, and walking. International Conference on Infant Studies, Brighton, England. Lo, T., Avolio, A. M., Massop, S. A., & Adolph, K. E. (1999). Why toddlers don't perceive risky ground based on surface friction. In M. A. Grealy & J. A. Thompson (Eds.), Studies in Perception and Action V (pp. 231-235). Mahwah, NJ: Erlbaum. Lockman, J. J. (1984). The development of detour ability during infancy. Child Development, 55, 482-491. McCaskill, C. L., & Wellman, B. L. (1938). A study of common motor achievements at the preschool ages. Child Development, 9, 141-150. McCollum, G., & Leen, T. K. (1989). Form and exploration of mechanical stability limits in erect stance. Journal of Motor Behavior, 21, 225-244. McGraw, M. (1935). Growth: A study of Johnny and Jimmy. New York: Appleton-Century.

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McGraw, M. (1945). The neuromuscular maturation of the human infant. New York: Columbia University Press. McKenzie, B. E., Skouteris, H., Day, R. H., Hartman, B., & Yonas, A. (1993). Effective action by infants to contact objects by reaching and leaning. Child Development, 64, 415-429. Metcalfe, J. S., & Clark, J. E. (2000). Sensory information affords exploration of posture in newly walking infants and toddlers. Infant Behavior and Development, 23, 391-405. Nashner, L. M., & McCollum, G. (1985). The organization of human postural movements: A formal basis and experimental synthesis. Behavioral and Brain Sciences, 8, 135-172. Palmer, C. E. (1944). Studies of the center of gravity in the human body. Child Development, 15, 99-163. Palmer, C. E (1987, April). Between a rock and a hard place: Babies in tight spaces. Poster presented at the meeting of the Society for Research in Child Development, Baltimore, MD. Piaget, J. (1954). The construction of reality in the child. New York: Basic Books. Rader, N., Bausano, M., & Richards, J. E. (1980). On the nature of the visual-cliff-avoidance response in human infants. Child Development, 51, 61-68. Reed, E. S. (1982). An outline of a theory of action systems. Journal of Motor Behavior, 14, 98134. Riccio, G. E. (1993). Information in movement variability about the qualitative dynamics of posture and orientation. In K. M. Newell & D. M. Corcos (Eds.), Variability and Motor Control (pp. 317-357). Champaign, IL: Human Kinetics. Riccio, G. E., & Stoffregen, T. A. (1988). Affordances as constraints on the control of stance. Human Movement Science, 7, 265-300. Richards, J. E., & Rader, N. (1983). Affective, behavioral, and avoidance responses on the visual cliff: Effects of crawling onset age, crawling experience, and testing age. Psychophysiology, 20, 633-642. Rochat, E (1992). Self-sitting and reaching in 5- to 8-month-old infants: The impact of posture and its development on early eye-hand coordination. Journal of Motor Development, 24, 210-220. Rochat, E, & Goubet, N. (1995). Development of sitting and reaching in 5- to 6-month-old infants. Infant Behavior and Development, 18, 53-68. Rochat, E, & Senders, S. J. (1991). Active touch in infancy: Action systems in development. In M. J. Weiss & E R. Zelazo (Eds.), Biological constraints and the influence of experience (pp. 412-442). Norwood NJ: Ablex. Scarr, S., & Salapatek, E (1970). Patterns of fear development during infancy. Merrill-Palmer Quarterly, 16, 53-90. Schmidt, R. A. (1988). Motor control and learning: A behavioral emphasis. Champaign, IL: Human Kinetics. Schmuckler, M. A. (1996). Development of visually guided locomotion: Barrier crossing by toddlers. Ecological Psychology, 8, 209-236. Schmuckler, M. A., & Gibson, E. J. (1989). The effect of imposed optical flow on guided locomotion in young walkers. British Journal of Developmental Psychology, 7, 193-206. Shumway-Cooke, A., & Woollacott, M. H. (1985). The growth of stability: Postural control from a developmental perspective. Journal of Motor Behavior, 17, 131-147. Slater, A., & Morison, V. (1985). Shape constancy and slant perception at birth. Perception, 14, 337344. Stoffregen, T., Adolph, K. E., Thelen, E., Gorday, K. M., & Sheng, Y. Y. (1997). Toddlers' postural adaptations to different support surfaces. Motor Control, 1, 119-137. Stoffregen, T. A., & Riccio, G. E. (1988). An ecological theory of orientation and the vestibular system. Psychological Review, 95, 3-14. Stoffregen, T. A., Schmuckler, M. A., & Gibson, E. J. (1987). Use of central and peripheral optical flow in stance and locomotion in young walkers. Perception, 16, 113-119.

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S E X U A L S E L E C T I O N A N D H U M A N LIFE H I S T O R Y

David C. Geary DEPARTMENT OF PSYCHOLOGICAL SCIENCES UNIVERSITY OF MISSOURI AT COLUMBIA COLUMBIA, MISSOURI 65211

I. INTRODUCTION II. N A T U R A L SELECTION AND LIFE HISTORY A. N A T U R A L SELECTION B. LIFE HISTORY III. SEXUAL SELECTION A. M A T I N G OR PARENTING? B. I N T R A S E X U A L COMPETITION C. INTERSEXUAL CHOICE IV. LIFE HISTORY AND SEXUAL SELECTION A. I N T R A S E X U A L COMPETITION B. INTERSEXUAL CHOICE C. PHENOTYPIC PLASTICITY V. H U M A N D E V E L O P M E N T A L SEX DIFFERENCES A. SEXUAL SELECTION DURING H U M A N EVOLUTION B. SEX DIFFERENCES IN LIFE HISTORY C. SEX DIFFERENCES IN D E V E L O P M E N T A L ACTIVITY VI. CONCLUSION REFERENCES

I. Introduction What is the evolutionary raison d'etre of lifetimes and effort? w R . D. Alexander, The biology ofmoral systems (1987, p. 38)

Sexual selection and life history are firmly established disciplines in evolutionary biology, and associated theory and research are focused on determining the ultimate and proximate causes of sex differences and developmental patterns, respectively 41 ADVANCESIN CHILDDEVELOPMENT AND BEHAVIOR,VOL.30

Copyright 2002, Elsevier Science (USA). All rightsreserved. 0065-2407/02 $35.00

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(Andersson, 1994; Charnov, 1993; Darwin, 1871; Roff, 1992). Research in human developmental science in general and human developmental sex differences in particular has not been informed by this wealth of empirical and theoretical work, with a few exceptions (Archer, 1992; Bjorklund & Pellegrini, 2002; Bogin, 1999; Freedman, 1974; Hill & Kaplan, 1999; Kenrick & Luce, 2000). As a redress, the general focus here is on relating human developmental sex differences to sexual selection and life history (Geary, 1999). In particular, I describe theory and evidence regarding the view that many human life history traits and developmental sex differences have evolved as a result of various forms of social competition (Alexander, 1987, 1989; Geary & Flinn, 2001). To provide the necessary background and to introduce human developmental scientists to relevant work in evolutionary biology, in the next three sections I review research on life history, sexual selection, and their relation in nonhuman species. Then I relate the basic patterns and principles described in the first three sections to human developmental sex differences, with a focus on the relation between the social competition inherent in the dynamics of sexual selection and the evolution of sex differences in human life history traits (e.g., adult size, maturational patterns) and in developmental activity (e.g., play).

II. Natural Selection and Life History I begin by describing the basic mechanisms of natural selection along with the basic principles of life history and sexual selection. Then I meld the principles of life history and sexual selection to provide the theoretical foundation for interpreting research on human developmental sex differences. A. NATURALSELECTION The fundamental observations and inferences that led to Darwin's and Wallace's (1858; Darwin, 1859) insights regarding natural selection and evolutionary change are shown in Table I. Of particular importance are individual differences, which largely result as a consequences of sexual reproduction (Hamilton & Zuk, 1982; Williams, 1975). The process of natural selection occurs when heritable variability in a trait, such as age of reproductive maturity, covaries with variability in survival or reproductive outcomes (Price, 1970). As an example, if age of maturation is heritable and early maturing individuals survive to maturity and thus reproduce more successfully than later maturing individuals, then after many generations the mean age of maturation for this population will shift downward (see Reznick & Endler, 1982). The strength of selection pressures can vary such that individual differences in some traits strongly influence the probability of survival or reproduction (e.g., to next breeding season), whereas other traits are only weakly related to or are

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TABLE I Darwin's and Wallace's Observations and Inferences Observations

Inferences

1. All species have such high potential fertility that populations should increase exponentially. 2. Except for minor annual and rare major fluctuations, population size is typically stable. 3. Natural resources are limited and in a stable environment they remain constant. 1. More individuals are born than can be supported by available resources, resulting in competition for those resources that covary with survival prospects. 1. No two individuals are exactly the same; populations have great variability. 2. Much of this variability appears to be related to inheritance that is passed on from parents to offspring. 1. Prospects for survival are not entirely random but covary with inherited characteristics. The relation between these characteristics and differential survival is natural selection. 2. Over generations, natural selection leads to gradual change in the population, that is, microevolution, and production of new species, that is, macroevolution or speciation.

Note: Observations and inferences are based on Darwin and Wallace (1858), Darwin (1859), and Mayr (1982). Although genetics were not yet understood, Darwin inferred that traits were passed on from parent to offspring through, among other things, what was known about the effects of selective breeding (artificial selection) on the emergence of various domestic species.

unrelated to survival or reproductive prospects. If strong selection is maintained across many generations, then heritable variability should be reduced to zero, and it has been for some traits (e.g., all genetically normal humans have two legs, a heritable trait that shows no variability across individuals). However, for a variety of reasons many of the traits that covary with survival and reproductive outcomes show heritable variability and are thus subject to evolutionary change (see Roff, 1992, for a discussion of why heritable variability is maintained). Mousseau and Roff (1987) conducted a comprehensive review of the heritable variability of the morphological, behavioral, physiological, and life history phenotypes (i.e., measurable traits) that covary with survival and reproductive outcomes in wild,

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outbred animal populations. The analysis included 1120 heritability estimates-the proportion of variability across individuals that appears to be due to genetic variability--across 75 invertebrate and vertebrate species. Although there was considerable variation--across species, contexts, and phenotypes--in the magnitude of the heritability estimate, their analysis indicated that "significant genetic variance is maintained within most natural populations, even for traits closely affiliated with fitness" (Mousseau & Roff, 1987, p. 188). The median heritability estimates were .26 for life history traits (e.g., age of maturation), .27 for physiological traits (e.g., cardiovascular capacity), .32 for behavioral traits (e.g., mating displays), and .53 for morphological traits (e.g., body size), values that are similar to those found in human populations (Plomin, DeFries, McClearn, & McGuffin, 2001). Kingsolver and colleagues (2001) reviewed field studies of the relation between the types of traits analyzed by Mousseau and Roff (1987) and survival and reproductive outcomes in wild populations (see also Endler, 1986). Across species and triats, the median effect size indicated that being one standard deviation above (e.g., late maturation) or below (e.g., early maturation) the mean was associated with a 16% increase in survival (e.g., surviving to next breeding season) or reproductive (e.g., number of offspring) fitness. If the heritability of any such trait was only .25, "then selection of this magnitude would cause the trait to change by one standard deviation in only 25 generations" (Conner, 2001, p. 216), or in 12-13 generations with a heritability of .50. The basic point is that the principles of natural selection have been empirically evaluated in many species and for many different traits. Many of these traits both show heritable variability and covary with survival and reproductive outcomes, the conditions needed for natural selection and thus evolutionary change to occur (see Table I). B. LIFE HISTORY

As aptly described by Alexander, "lifetimes have evolved to maximize the likelihood of genic survival through reproduction" (Alexander, 1987, p. 65), and the focus of life history research is on the suite of phenotypic traits that defines the species' maturational and reproductive pattern (Charnov, 1993; Roff, 1992). A suite of traits must be considered because of the trade-offs involved in the expression of one phenotype versus another (Williams, 1957). The trade-offs are commonly conceptualized in terms of a competitive allocation of resources (e.g., calories) to somatic effort or reproductive effort, as shown in Figure 1 (Alexander, 1987; Reznick, 1985, 1992; Williams, 1966). Somatic effort is traditionally defined as resources devoted to physical growth and to maintenance of physical systems during development and in adulthood (see West, Brown, & Enquist, 2001), although growth also involves the accumulation, as in increases in body size, of reproductive potential. Reproductive effort is expended during adulthood and is distributed

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Life History Somatic Effort . _

_

.

Infancyand Juvenility Growth

.

Reproductive Effort .

.

_

Developmental Activity

Life Span Maintenance (Survival)

Adult ReproductiveYears Mating

Parenting

Nepotism

Fig. 1. Components of life history. Development activity refers to social, behavioral and cognitive activities during juvenility that promote survival and increase reproductive potential (see Figure 6). Nepotism refers to activities that promote the somatic or reproductive efforts of kin, such as nephews and nieces.

among mating, parenting, and in some species nepotism, that is, investment in kin other than offspring (Emlen, 1995; Hamilton, 1964). In addition to these traditional components of life history, Figure 1 includes developmental activity as a feature of somatic effort during infancy and juvenility. Some developmental activities will promote survival during development (e.g., predator avoidance), whereas others are analogous to the relation between physical growth and the accumulation of reproductive potential. The latter developmental activities result in the refinement of behavioral (e.g., practicing mating displays), cognitive (e.g., birdsong), and physical (e.g., improving cardiovascular capacity) competencies that will later influence reproductive prospects (Geary & Bjorklund, 2000), and presumably result in somatic changes (e.g., modification of neural systems supporting birdsong) during infancy and juvenility. In other words, the results of many developmental activities are incorporated into the developing soma--for instance, distribution of slow and fast muscle fibers as related to physical activity (Byers & Walker, 1995)--and facilitate later reproductive activities. Lifetimes are thus conceptualized as involving the accumulation of reproductive potential--captured by growth and developmental activity during infancy and juvenility--and then the expenditure of this potential on reproductive effort in adulthood (Alexander, 1987). The clearest examples of this view of lifetimes are found in many species of Insecta where distinct morphs are associated with different life history stages. An illustration is provided in Figure 2 for the tomato hornworm moth (Manduca quinquemaculata), where the behavior of the larvae (caterpillars) is focused on somatic effort--to avoid predation and to growmbut the behavior of the adult (moth) is focused on reproductive effort. In fact, the caterpillar morph cannot reproduce and in some species of Insecta the adult morph does not eat; that is, the sole function of the moth or butterfly is to reproduce (Alexander, 1987). Although life history traits will sometimes fall into more than one of the categories shown in Figure 1--sex hormones, for instance, may influence growth as

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Fig. 2. Two life history stages of the tomato hornworm moth (Manduca quinquemaculata). To the left is the larval stage during which the caterpillar's behavior is focused on somatic effort, that is, avoiding predation and growth. To the right is the adult stage during which the moth's behavior is focused on reproductive effort. United States Department of Agriculture, public domain illustrations.

well as allocation of reproductive effort into mating or parenting--these categories nonetheless provide a useful heuristic for conceptualizing trade-offs. Within finite lifetimes, trade-offs between the different components of somatic and reproductive effort can be conceptualized in terms of the relative size of the corresponding rectangles in Figure 1. As examples, traits that facilitate predator avoidance (e.g., dull coloration, a feature of somatic effort) may be negatively correlated with mating success (e.g., attracting mates), or traits that facilitate mating activities (e.g., testosterone) may negatively affect health (Folstad & Karter, 1992). Thus the selective advantage for expressing one trait often has an associated cost in terms of other selection pressures or in terms of the expression of other traits (Williams, 1966). Because of these trade-offs, selection pressures do not commonly operate such that a single trait is optimally expressed (e.g., having the brightest possible plumage within physiological constraints). Rather, selection will result in an evolved combination of traits that minimize costs (e.g., predation risks) and enable--within trade-off and ecological (e.g., food) constraints--producing the optimal number of offspring that are likely to survive to adulthood and reproduce themselves (Roff, 1992). A full discussion of the complexity of life history trade-offs is beyond the scope of this treatment (see Charnov, 1993; Roff, 1992; Stearns, 1992), but in the pages that follow I describe some of the most basic of these trade-offs as well as the related issue of phenotypic plasticity.

1. Lifetime Pattern of Reproduction Reproductive activity takes on two general forms: in semelparity all reproductive potential is spent in one breeding episode, but in iteroparity reproductive potential is allocated across more than one breeding episode. Semelparity is a more risky strategy because reproduction during poor ecological conditions could result in extremely high offspring mortality rates, with no opportunity to reproduce under

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more favorable conditions. Semelparity is, however, favored when adult mortality is high and thus the probability of surviving to the next breeding season is low. Under these conditions, individuals that devote minimal resources to somatic effort in adulthood and maximal resources to reproductive effort will produce more offspring than individuals that do not. In contrast, iteroparity is favored when juveniles and adults are likely to survive from one breeding season to the next (e.g., due to low predation risks) and juveniles are unlikely to reproduce successfully (Roff, 1992; Wittenberger, 1979). For these species, the current reproductive effort is balanced against the costs of this effort with respect to survival and future reproductive potential. As a result, during each breeding season iteroparous species invest more in maintenance and less in reproduction than semelparous species (Roff, 1992). A comparison of female Pacific salmon (Salmo oncorhynchus) and female Atlantic salmon (S. salar) illustrates how selection pressures can influence the evolution of a semelparous or iteroparous reproductive strategy. Female Pacific salmon experience intense competition for suitable nesting sites and must guard these sites after depositing their eggs (de Gaudemar, 1998). The intensity of the competition favors expending all resources in one reproductive episode. Females that do not incur the costs of competition will not obtain a suitable nesting site or will have their site destroyed by other females, and thus they will not reproduce at all. The result of this competition has been the evolution of a life history strategy such that resources that could be used for maintenance and survival--females die at the end of the first breeding episode--are expended on behavioral competition for nesting sites and on the development of eggs. The latter results in the production of several-fold more eggs than the iteroparous Atlantic salmon (Roff, 1992). In contrast, competition among females for suitable nesting sites is less intense in Atlantic salmon. In this species, females devote more resources to maintenance and less to reproduction during each breeding season and thus survive to reproduce over many breeding seasons (Roff, 1992). The advantage of distributing reproduction over several seasons is to counter year-to-year fluctuations in predation or other risks (e.g., lack of food) to offspring survival. Although female Atlantic salmon produce fewer eggs during any single season than do female Pacific salmon, the number of viable offspring produced during the reproductive life span of these two species is comparable. There are also variations in life history traits (e.g., age of maturation) across semelparous species and across iteroparous species. These cross-species differences are understandable in terms of differences in selection pressures in each species' specific ecology. As an example, consider Reznick and Endler's (1982) study of the influence of predation on growth and reproductive patterns in iteroparous guppies (Poecilia reticulata). Here, three populations of the same species were studied under three patterns of predation risk: high risk (predators feeding on large adults), medium risk (predators feeding on juveniles), and low risk (few predators). When risk was high, females matured more rapidly and were smaller

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as adults, two factors that lowered their risk of being eaten by predators before reproducing. In addition, they allocated more reproductive effort to initial breeding episodes, producing two to three times as many offspring in these breeding episodes than did females in less risky environments. In locales where predation was less severe and adult mortality rates were lower, individuals grew more slowly, attained a larger adult size, and females allocated their reproductive effort over more breeding episodes. Follow-up studies revealed that these differences in life history pattern were due to a combination of genetic differences between these populations and phenotypic plasticity (discussed later, Reznick & Bryga, 1987, 1996; Reznick, Shaw, Rodd, & Shaw, 1997; Rodd, Reznick, & Skolowski, 1997). In sum, Reznick and colleagues' studies empirically demonstrate systematic relations between predation risks and life history and reproductive parameters in three populations of the same species of guppy and thus illustrate the conditions that could ultimately lead to the emergence of different species of guppy with different life history and repoductive traits (Darwin, 1859).

2. Reproductive Costs Reproduction involves costs associated with mating (e.g., finding mates), producing gametes and offspring (e.g., eggs), and for many species parental care (Roff, 1992). Mechanisms underlying the cost~enefit trade-offs involved in reproducing may be genetic or social/environmental, or they may represent a genotype by environment interaction (Reznick, Nunney, & Tessier, 2000). Social costs include those incurred during intrasexual competition over mates and are described later. Genetic trade-offs arise when the same gene or genes affect two or more life history traits (Williams, 1957). In many species, reproducing earlier in life is associated with a shorter life span (Reznick, 1992). The same genes that promote early reproduction have the negative consequence of accelerating the onset of senescence and reducing the life span. Life span is also influenced by more proximal reproductive costs, such as producing eggs, competing for mates, and caring for offspring, which can compromise the physical health and oftentimes the survival prospects of parents (Clutton-Brock, 1991; Steams, 1992). The underlying physiological mechanisms governing these cost/benefit trade-offs are not fully understood, but include the energetic demands of reproduction (e.g., parental care) and associated hormonal changes (Sinervo & Svensson, 1998). For example, the development of male secondary sexual characteristics needed to compete with other males (e.g., antlers) or to attract females (e.g., a bright plumage) requires an increase in testosterone levels which in turn can compromise the immune system and survival prospects of unhealthy males (Folstad & Karter, 1992; Saino & Mr 1994; Saino, Mr & Bolzern, 1995). Similarly, in the female collard flycatcher (Ficedula albicollis) large brood sizes are associated with a reduced production of antibodies for a common parasite; the result is increased infection rate and mortality rate (Nordling, Andersson, Zohari, & Gustafsson, 1998).

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3. Growth and Development All other things being equal, evolution should result in a life history pattern in which females produce many, fast maturing offspring, that have an increased probability of surviving to reproduce (Williams, 1966). The fact that many species do not show this life history pattern indicates that the associated trade-offs are costly. These trade-offs include smaller and less competitive offspring that in turn suffer high mortality rates (Stearns, 1992). Across species of plant, insect, fish, reptile, and mammal, offspring that are larger at time of hatching or birth have increased survival rates due, in part, to decreased predation risk and decreased risk of starvation (Roff, 1992). The trade-off is that females of these species produce fewer offspring than do females of related species that produce many smaller offspring. Thus fast maturation and large numbers of offspring are associated with low-quality offspring (i.e., high mortality risks and low competitiveness). High-quality--larger and more competitiveuoffspring come at a cost of fewer offspring produced during a reproductive life span. Many factors will influence whether a species tends toward a low-quality/high-quantity or high-quality/lowquantity reproductive pattern, including age-specific mortality risks (e.g., through predation), population stability or expansion, and intensity of competition with conspecifics (Mac Arthur & Wilson, 1967; Steams, 1992; Roff, 1992). Species that produce fewer and larger offspring also tend to have slower rates of growth, higher levels of parental care, and longer life spans in comparison to related species that produce smaller but more offspring (Roff, 1992; Shine, 1978, 1989; Stearns, 1992). This life history pattern is more common in iteroparous than in semelparous species and is associated with relatively low juvenile mortality rates and a low probability of reproducing at an early age (Roff, 1992). Low juvenile mortality is related to larger size at hatching or birth as well as to parental protection and provisioning (Clutton-Brock, 1991; Shine, 1978). As described later, a low probability of reproducing at an early age can result from reproductive competition with more mature individuals in the population. In this situation, delayed maturation can improve reproductive prospects through, for instance, an increase in body size. Large body size enables females to give birth to larger and thus more competitive offspring, and for males it facilitates malemale competition in adulthood (Carranza, 1996; Steams, 1992). In some species, developmental activity during the maturational period enables improvements in survival- and reproduction-related behavioral/cognitive competencies. Slow maturation and growth thus allows for the accumulation of more reproductive potential, through physical development and developmental activity, than is possible with faster maturing species. 4. Phenotypic Plasticity Phenotypic plasticity refers to the potential for the modification of survival- and reproduction-related phenotypes in response to social and ecological (e.g., food) conditions, but within genetically based constraints (Roff, 1992). The potential

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to modify the expression of life history traits presumably evolved as an adaptation to variability across seasons and generations in the ecologies in which the species evolved. Phenotypic plasticity enables a more optimal expression of life history traits as these relate to survival and reproductive demands in the local ecology. The mechanisms associated with plasticity include hormonal and/or other endocrine responses as well as ecological conditions (e.g., water availability) that affect the physical and behavioral condition of the individual (McNamara & Houston, 1996; Sinervo & Svensson, 1998). Phenotypic plasticity has been empirically demonstrated in a wide range of plant species (Fenner, 1998; Sultan, 2000) as well as in a diversity of other species ranging from plankton to primates (Alberts & Altmann, 1995; McLaren, 1966; McNarnara & Houston, 1996; Miaud, Guyrtant, Elmberg, 1999; Roff, 1992). In all of these species, phenotypic plasticity is expressed within the constraints of norms of reaction (Stearns & Koella, 1986). Norms of reaction represent a genotype whose phenotypic expression varies with ecological conditions, but only within a genetically constrained range. Consider field voles as one example (Microtus agrestis; Ergon, Lambin, & Stenseth, 2001). In this species, populations residing in different locales vary significantly in two life history traits, adult body mass and timing of yearly reproduction. On one hand, if the population differences reflect genetic variance then individuals transplanted from one population to the other will show the body mass and reproductive timing of their natal group. On the other hand, if the population differences reflect variation in local ecologies, such as quality and availability of food, then, in the season following transplantation, body mass and reproductive timing of transplanted individuals should be the same as that of the local community. In fact, the life history traits of transplanted individuals were indistinguishable from those of the local community and differed significantly from those of their natal community. Regardless of natal community, individuals living in richer ecologies developed a higher wintering body size and as a result were able to reproduce earlier. Individuals living in poorer ecologies needed to devote added time to foraging and growth--somatic effortuand thus experienced a delay in the onset of reproductionJreproductive effort. Phenotypic plasticity in growth and reproductive timing has also been demonstrated for many other species, including humans (Steams & Koella, 1986), as well as for many other life history traits (Roff, 1992). For some species, crossgenerational plasticity has been demonstrated, whereby the ecological conditions experienced by the mother influence life history trade-offs in offspring (Hofer, 1987). For example, offspring of nutrient-deprived plants allocate more growth-related resources to root production, whereas offspring of light-deprived plants allocate more resources to leaf production (Sultan, 2000; see also Alekseev and Lampert, 2001, for an analogous mechanism in the crustacean Daphnia). In mammals, maternal condition during pregnancy and during offspring suckling can

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have long-term reproductive consequences. Healthy mothers give birth to heavier offspring and they provide more milk, both of which promote early growth and this, in turn, is associated with larger adult size and higher breeding success (CluttonBrock, 1991). As an example involving social dynamics, testicular maturation and achievement of social dominance are accelerated in male baboons (Papio cynocephalus) borne to high-ranking females, thereby enhancing the males' reproductive prospects (Alberts & Altmann, 1995). 5. Conclusion

From plankton to primates, considerable empirical evidence supports the position that age of maturation, reproductive pattern, number of offspring, extent of parenting, and length of the developmental period are evolved features of the species' life history (Williams, 1966; Roff, 1992; Steams, 1992). The accompanying suite of developmental and reproductive traits essentially involves the respective accumulation and expenditure of reproductive potential, within the constraints imposed by external conditions, such as parasites and predators, and social competition, including sexual selection (Alexander, 1987).

III. Sexual Selection Sexual selection refers to the processes associated with mating competition with members of the same sex and species (intrasexual competition) and the processes associated with choosing mates (intersexual choice; Darwin, 1871). Sexual selection is related to sex differences in hundreds of species and most typically includes male-male competition over access to mates and female choice of mating partners (Andersson, 1994). As I describe in the first part of this section, the dynamics of sexual selection turn on the degree to which members of each sex allocate their reproductive effort to competing for mates or investing in parenting. In the second and third parts of this section, I illustrate the evolutionary influences of intrasexual competition and intersexual choice, respectively. A. MATINGOR PARENTING? As shown in Figure 1, reproductive effort is distributed between mating (e.g., time spent searching for mates), parenting, and occasionally nepotism. Nepotism is less central to the later discussion of human life history and is not considered further (see Emlen, 1995); this is not to say that humans do not engage in significant levels of nepotism--they do (see Geary & Flinn, 2001; Pasternak, Ember, & Ember, 1997). The distribution of reproductive effort across mating and parenting is, however, central to the later discussion, and it turns on the extent of each sexes' parental effort or parental investment (Trivers, 1972; Williams, 1966).

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Parental investment is any cost (e.g., time) associated with raising offspring that reduces the parents' ability to produce or invest in other offspring (Trivets, 1974). Given that some level of parental investment (even if it only involves producing eggs) is necessary for the reproduction of both parents, the nature of the investment provided by females and males creates the basic dynamics of sexual reproduction and sexual selection. If one sex provides more than his or her share of parental investment, then members of that sex become an important reproductive resource for members of the opposite sex (Dawkins, 1989; Trivers, 1972). The reproductive success of members of the lesser investing sex is more strongly influenced by the number of mates that can be found than by investing in the well-being of individual offspring, whereas the reproductive success of members of the more highly investing sex is more strongly influenced, in most cases, by investment in offspring than in finding mates. In most species, the sexes differ in the degree to which the reproductive effort is allocated to competition for access to mates or to parental investment (Andersson, 1994; Trivers, 1972; Williams, 1966). These differences, in turn, are related to the potential rate of reproduction and to social and ecological influences on mating opportunities, in particular, the operational sex ratio (OSR; Clutton-Brock & Vincent, 1991; Emlen & Oring, 1977; Krebs & Davies, 1993). Reproductive rates and the OSR are related but described separately.

1. Reproductive Rates A sex difference in potential rate of reproduction can create a sex difference in relative emphasis on mating or on parenting. Most generally, the sex with the higher potential rate of reproduction invests more in mating effort than in parental effort, whereas the sex with the lower rate of reproduction invests more in parental effort than in mating effort (Clutton-Brock & Vincent, 1991). This pattern arises because members of the sex with the higher potential rate of reproduction can rejoin the mating pool more quickly than can members of the opposite sex. Under these conditions, individuals of the sex with the faster rate of reproduction will typically have a higher lifetime reproductive success if they rejoin the mating pool and compete for mates than if they parent (Parker & Simmons, 1996). For species with internal gestation and obligatory postpartum female care (e.g., suckling in mammals), the rate at which females can produce offspring is considerably lower than the potential rate of reproduction of conspecific males (CluttonBrock, 1991). In addition, internal gestation and the need for postnatal care results in a strong bias in mammalian females toward parental investment and results in a sex difference in the benefits of seeking additional mates (Trivers, 1972). Males can benefit, reproductively, from seeking and obtaining additional mates, whereas females cannot. In other words, males that compete for additional mates typically have more offspring than do males that do not compete and instead invest in parenting. Thus, the sex difference in reproductive rate, combined with offspring that can

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be effectively raised by the female, creates the potential for a large female-male difference in the mix of mating and parenting, and this difference is realized in 95-97% of mammalian species (Clutton-Brock, 1989). In these species, females can effectively provide the majority of parental care and do so. Female care, in turn, frees males to invest in mating effort, which typically takes the form of male-male competition over access to mates or for control of the resources (e.g., territory) that females need to raise their offspring.

2. Operational Sex Ratio The OSR is defined as the ratio of sexually active males to sexually active females in a given breeding population at a given point in time and is related to the rate of reproduction (Emlen & Oring, 1977). In a population where the number of sexually mature females equals the number of sexually mature males--an actual sex ratio of l : l - - a n y sex difference in the rate of reproduction will skew the OSR. As noted, mammalian males have a faster potential rate of reproduction than conspecific females, which typically results in more sexually receptive males than sexually receptive females in most populations. This biased OSR creates the conditions that lead to intense male-male competition over access to a limited number of potential mates. Although these patterns are most evident in mammals, they are also found in many species of bird, fish, and reptile (Andersson, 1994). And they are not limited to males: When females have a faster rate of reproduction than males (e.g., when males care for eggs), female-female competition is often more salient than male-male competition (e.g., Reynolds, 1987). The sex difference in potential reproductive rate and a skewed OSR appear to be the ultimate sources of the male focus on mating effort and the female focus on parental effort in the vast majority of mammalian species (Emlen & Oring, 1977; Parker & Simmons, 1996). The biology of internal gestation and suckling are not the only factors that influence the potential rate of reproduction and the OSR in mammals; social and ecological factors are sometimes important as well. As an example, male callitrichid monkeys (Callithrix) have a higher potential rate of reproduction than conspecific females do. However, shared territorial defense, female-on-female aggression that drives away the males' potential mating partners, and other factors negate this physiologically based sex difference and result in a more balanced OSR, monogamy, and high levels of paternal investment (see Dunbar, 1995). B. INTRASEXUAL COMPETITION

Intrasexual competition over mates, whether male-male competition or femalefemale competition, will result in the evolutionary emergence of sex differences for those traits that facilitate this competition (Andersson, 1994; Darwin, 1871). Studies of intrasexual competition have revealed that the associated sex differences

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can be physical, behavioral, or cognitive (including neural) and typically only affect those features actually involved in the competition (Andersson, 1994; Geary, 1998). One of the more common expressions of intrasexual competition involves physical threats and fights over access to mates or for control of the territory that members of the opposite sex need to raise offspring (e.g., nesting spots). A common result is that physically larger, healthier, and more aggressive individuals (typically males) monopolize the reproductive potential of members of the opposite sex (typically females). The accompanying individual differences in reproductive success--some individuals have many offspring, others have few or none--result in the evolution of sex differences in physical size and aggressiveness. The polygynous ruff (Machetes pugnax) provides one example of such competition among males. As described by Darwin, males are considerably larger and more aggressive than females and physically compete for sexual access to females. These physical and behavioral sex differences evolved in the ruff, and many other species, because of the reproductive advantages associated with larger size and pugnacity in males (Darwin, 1871). Sometimes the competition is more behavioral or cognitive (e.g., spatial cognition, as in searching for mates) than physical (Gaulin & Fitzgerald, 1986, 1989; Gilliard, 1969). In these situations, behavioral and cognitive traits that facilitate intrasexual competition will evolve in the same way that physical traits evolve (see Geary, 1998, for elaboration). When males parent (e.g., incubate eggs), females may compete more intensely for mates than males; that is, physical female-female competition is more intense than male-male competition. In these species, females are larger and more aggressive than males (see Reynolds, 1987). c. INTERSEXUALCHOICE The sex that invests more in parenting tends to be more choosy with regard to mating partners than the other sex (Trivers, 1972). Because females tend to invest more in parenting than males, female choice is predicted to be and is more common than male choice. Male choice is predicted for species with paternal investment, although this prediction has not been as thoroughly tested as female choice. In any case, female choice has been studied most extensively in birds, although it is also evident in insects, fish, reptiles, and mammals, including humans (Andersson, 1994; Buss, 1994). Several examples of male traits that have been shaped by female choice are shown in Figures 3 and 4; in some species these traits may also be involved in male-male competition, as in dominance displays. These traits often involve elaborate physical displays, as in the crest along the back and tail of the male crested newt (Triturus cristatus; Figure 3), the dorsal fin of the male dragonet (Callionymus lyra; Figure 3), and the comb of the male hoopoe (Upupa epops; Figure 4). In many species, males are often more elaborately colored than females. The comb of the male hoopoe is a bright orange, and the male dragonet has a brilliant yellow body of varying shades, whereas the female dragonet is a "dingy reddish-brown" (Darwin, 1871, Vol. II, p. 8).

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Fig. 3. Indicators of male fitness shaped by female choice for selected species of amphibian and fish. To the left are male (top) and female (bottom) Triturus cristatus (from The Descent of Man, and Selection in Relation to Sex, Part II, p. 24, by C. Darwin, 1871, London: John Murray). To the right are male (top) and female (bottom) Callionymus lyra (from The Descent of Man, and Selection in Relation to Sex, Part II, p. 8, by C. Darwin, 1871, London: John Murray).

Traits such as those shown in Figure 3 are indicators of the physical, genetic, or behavioral fitness (e.g., ability to provide) of the male. These traits are honest indicators of male fitness, as they commonly are not expressed in unfit males (Zahavi, 1975). As an example, in some species of bird the coloration of male plumage covaries with physical health, in particular, resistance to infection by local

Fig. 4. The size and coloration of the comb of the male hoopoe (Upupa epops) are indicators of male fitness shaped by female choice (from A History of British Birds, p. 50, by F. O. Morris, 1891, London: Nimmo ).

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parasites (see Hamilton & Zuk, 1982). In red jungle fowl chicks (Gallus gallus), males infected with a parasitic worm (Ascaridia galli) grow more slowly than their healthy peers, and, in adulthood, their sexually selected characteristics are more adversely affected by the infection than are other physical characteristics. For instance, the comb of affected males is smaller and duller than that of unaffected males but many other physical characteristics (e.g., ankle length) do not differ across these groups. Furthermore, females substantially prefer unaffected males (by two to one), and preference is related to sexually selected features, such as comb length, but not other features, such as ankle length (Sheldon, Meril~i, Qvarnstrrm, Gustafsson, & Ellegren, 1997; Zuk, Thornhill, & Ligon, 1990). Related studies have demonstrated that unfit males cannot tolerate the hormonal changes needed for the expression of sexually selected traits. Experimentally increasing male testosterone levels to induce the expression of secondary sexual characteristics results in increased mortality in unhealthy males, perhaps due to suppression of immune functions (Folstad & Karter, 1992). In series of field experiments, Siano and colleagues assessed the effect of testosterone implants on mortality rates in the barn swallow (Hirundo rustica; Saino et al., 1995; Saino, Bolzern, & Moiler, 1997). In this species, female choice is influenced by the length and symmetry of the male's tail features (MOiler, 1994). Testosterone implantation suppresses the immune system in males with shorter tail feathers more severely than in males with longer tail feathers and results in increased parasite loads and higher mortality rates in shorter tailed than in longer tailed males. The pattern indicates that males with long tail feathers can support high testosterone levels--and thus more effectively compete for mates--without compromising their immune system, suggesting that their immune system is well adapted to local parasites or that they are in better general physical condition than are males with short tail feathers. The pattern also illustrates an important life history trade-off for shorter tailed males. Resources (e.g., calories) that could be used for the development of a sexually selected trait must be diverted to immune functions. The cost is an inability to attract mates during the current mating season, and the benefit is increased survival prospects and thus an opportunity to mate in subsequent seasons.

IV. Life History and Sexual Selection Sexual selection and life history have been linked theoretically and empirically, although the extent of the interrelation is not fully understood (Andersson, 1994). Predation risks, for example, may indirectly influence the opportunities for intrasexual competition or intersexual choice to operate and thus evolve (Partridge & Endler, 1987; Winemiller, 1992). More relevant to the current treatment is the prediction that social dynamics inherent in intrasexual competition and intersexual choice can influence and be influenced by life history traits (Hrglund &

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Sheldon, 1998; Kokko, 1997; Svensson & Sheldon, 1998). In fact, competition among members of the same species should, in theory, favor life history traits that include fewer, more competitive offspring (Mac Arthur & Wilson, 1967), which favors the evolution of iteroparity, a longer developmental period, higher levels of parental investment, and other traits that support social competition (Roff, 1992). Empirically, many of the hormonal mechanisms that influence the expression of life history traits (e.g., maturational timing) and trade-offs (e.g., mortality risks) also influence expression of secondary sexual characteristics involved in intrasexual competition and intersexual choice (Sinervo & Svensson, 1998). The full extent of the relation between sexual selection and life history remains to be determined. For now, in the parts that follow I illustrate how intrasexual competition and intersexual choice appear to relate to the evolution and phenotypic expression of life history traits and trade-offs. In the final part, I address the issue of phenotypic plasticity. A. INTRASEXUAL COMPETITION

Studies of the relation between intrasexual competition and life history have focused largely on males (Stearns, 1992), presumably because male-male competition is more common than female-female competition (Darwin, 1871; Andersson, 1994). 1 The relation between male-male competition and a few life history traits (e.g., age of maturation) have been studied in a variety of mammalian and bird species (e.g., Clinton & Le Boeuf, 1993; Harvey & Clutton-Brock, 1985; McElligott & Hayden, 2000; Rohwer, Fretwell, & Niles, 1980; Wiley, 1974) as well as in some other species (Stamps, 1995). I first illustrate the relation between physical male-male competition and sex differences in life history traits and then consider the relation between behavioral competition and sex differences.

1. Physical Competition Males of many species of insect, reptile, fish, bird, and mammal show little or no parental investment and compete intensely for access to females (Andersson, 1994; Clutton-Brock, 1989; Darwin, 1871). One result is that only a minority of males reproduce, thereby creating strong selection pressures for the evolution of traits that support competitive ability (e.g., Clinton & Le Boeuf, 1993; McElligott & Hayden, 2000; Plavcan & van Schaik, 1997a). Among these traits are physical size and aggressiveness such that larger, more aggressive males are 1When it occurs, female-female competition should relate to life history traits in many of the same ways as male-male competition. Social dynamics in polyandrous shorebirds, for instance, include intense female-female competition for access to males to brood their eggs. Although the issues have not been thoroughly studied, females of these species show some of the same life history patterns common for males of other species in which male-male competition is intense (Reynolds, 1987; Reynolds & Szrkely, 1997).

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typically more competitive than smaller, less aggressive males. In theory, these conditions could result in the evolution of growth rates such that males grow faster than females and achieve large size at progressively younger ages. However, the calories needed to achieve large size and the lost opportunity to practice fighting before reproductive maturity place formidable constraints on the evolution of such a life history pattern. A more common pattern is for males to grow more slowly and mature later than females and to engage in play fighting during juvenility (Smith, 1982; Stearns, 1992). As an example, the mating dynamics of primates are consistently related to sex differences in maturational patterns (e.g., duration, growth spurt) and physical size (Leigh, 1995). Polygynous species with physical male-male competition are characterized by consistent sex differences, whereby males grow more slowly and evidence both a longer period of rapid growth (i.e., the growth spurt), and a longer overall developmental period than females. The result is larger males than females. In these species, both males and females are physically aggressive, but male-onmale physical aggression is related to competition for mates and is more severe and deadly than female-on-female aggression, which is related to competition for food (Smuts, 1987). Further evidence that the sex differences in life history pattern are related to male-male competition comes from comparisons of evolutionarily related (i.e., having a recent common ancestor) monogamous and polygynous species. Intrasexual competition is less intense in monogamous species and thus the selective advantages for physical size and aggressiveness are considerably less relative to polygynous species (Clutton-Brock, Harvey, & Rudder, 1977). Among monogamous species of primate, the sexes rarely differ in adult size or maturational pattern (Leigh, 1995). For mammalian species in which physical male-male competition is found, the development period of males can range from moderately longer (e.g., 2.8 vs 3.5 years in the patas monkey, Erythrocebus patas) to more than twice as long as that of females (e.g., 3.0 vs 8.0 years in the northern elephant seal, Mirounga angustirostris; Le Boeuf & Reiter, 1988; Harvey & Clutton-Brock, 1985; Stearns, 1992). Males may weigh slightly more than females (e.g., 19% heavier in colobus monkeys, Colobus angolensis) or can weigh more than double that of females (e.g., 120% heavier in mandrills, Mandrillus sphinx, another monkey; Harvey & Clutton-Brock, 1985). Intrasexual competition and accompanying reductions in parental investment (Trivers, 1972) are also related to average length of the life span (Allman, Rosin, Kumar, & Hasenstaub, 1998). The lesser investing sex shows more intense intrasexual competition and has a shorter life span on average than does the more highly investing sex, whether the latter is female or male. The sex difference in life span appears to be a consequence of the higher nutritional demands needed to grow larger, the injuries associated with male-male competition, and the immunosuppressive effects of testosterone (Clinton & Le Boeuf, 1993;

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Clutton-Brock, Albon, & Guinness, 1985; Folstad & Karter, 1992). The advantage of physical size also creates a selective advantage for larger offspring and an accompanying increase in the size of females, but with the trade-off of fewer offspring (Carranza, 1996; Roff, 1992). The overall pattern is consistent with predictions noted earlier regarding social competition and life history evolution (Mac Arthur & Wilson, 1967; Roff, 1992). The features of primate life history and particularly that of humans (described later) and related species (e.g., chimpanzees, Pan troglodytes) appear to be especially in keeping with this hypothesis. Across species of primate, larger and fewer offspring are associated with the predicted life history patterns of longer interbirth intervals, higher levels of maternal investment, larger brains, longer developmental periods, and longer maximum life spans (Allman, McLaughlin, & Hakeem, 1993; Harvey & Clutton-Brock, 1985). Across these species, the length of the developmental period and brain size covary positively with the species' social complexity, the intensity of intrasexual competition, and with foraging complexity (Allman et al., 1993; Dunbar, 1993; Joffe, 1997; Sawaguchi, 1997), suggesting that a long maturational period is not simply about producing a larger body size, at least in primates. Presumably, a long developmental period and a large brain enable the practice and refinement of sociocompetitive and foraging skills~accumulation of reproductive potential~before engaging in actual (potentially life threatening) competition and unsupported (by parents) food acquisition.

2. Behavioral Competition Intrasexual competition with a strong behavioral component can result in alternative reproductive strategies and life histories for one or both sexes or in an exaggeration of the behavioral and associated life history traits. The different reproductive strategies and life histories of smaller jack and larger hooknose male salmon (Oncorhynchus kisutch) provide a clear example of the former (Gross, 1985). Hooknose males mature later and compete physically for access to eggs laid by females, whereas jack salmon are specialized to hide among rocks and furtively spawn while hooknose males are fighting. The smaller and earlier maturing jacks are just as reproductively successful, on average, as larger hooknose males, and thus early maturity and furtive mating represents a successful life history strategy for males of this species (Gross, 1985). Studies of bowerbirds provide some the best examples of how intrasexual competition and intersexual choice can involve behavioral competition and result in the evolution of behavioral sex differences (Gilliard, 1969). In most of these species, the principal focus of competition and choice is the bower, a structure made of tree boughs and vines shown in Figure 5 (Darwin, 1871). A female bowerbird's choice of mating partners is strongly influenced by the complexity and symmetry of the male's bower as well as by the number of decorations around the bower. Males thus compete with one another through bower building and through the destruction of

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Fig. 5. Bower building and behavioral male-male competition in the bowerbird (Chlamydera maculata) (from The Descent of Man, and Selection in Relation to Sex, Part II, p. 70, by C. Darwin, 1871, London: John Murray).

their competitors' bowers (Borgia, 1985a, 1985b; Collis & Borgia, 1992), although courtship displays, calls, and physical fights among males are also components of male-male competition and female choice (Borgia & Coleman, 2000). As with species in which physical male-male competition is prevalent, sex differences in the life history traits of one well-studied bowerbird, the satin bowerbird (Ptilonorhynchus violaceus), are evident, including differences in growth patterns and developmental activity. Female satin bowerbirds begin to reproduce at 2 years of age, whereas males do not produce sperm until they are 5 years of age and do not achieve an adult-male plumage until they are 6- or 7-years-old (Vellenga, 1980). Even then, most males that hold bowers do not mate until 10 years of age, if they mate at all (Borgia, personal communication, August 24, 2001). During development, young males watch older males at their bower and imitate bower building and courtship displays when the older male leaves the bower (e.g., to feed; Collis & Borgia, 1992). Young males also engage in play fighting, which provides the experience needed for dominance-related encounters in adulthood. Although the degree to which bower building is genetically or experientially based is not yet certain, the delayed maturation of male satin bowerbirds almost

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certainly provides an opportunity to practice and refine the bower construction and physical competition skills that will be needed in adulthood. In this circumstance, the later reproductive advantage of bower building and other competition-related skills will--when combined with low juvenile mortality risks--provide a selective advantage for delayed maturation in males. In other words, delayed maturation enables males through developmental activities to accumulate the reproductive potential needed to compete with other males and to attract female mates in adulthood. B. INTERSEXUALCHOICE The relation between intersexual choice--typically female choice--and sex differences in life history traits has received less attention than the relation between intrasexual competition and life history. One difficulty is that many traits that influence female choice are also related to male-male competition, as in dominance displays (e.g., birdsong often has this dual function; Borgia & Coleman, 2000). An important window into the likely influence of female choice on the evolution of life history traits in males comes from studies of lekking species (Htglund & Alatalo, 1995; Wiley, 1974). Leks can be areas in which males gather together during the mating season to strut, display plumage, or engage in other activities that function to attract mating partners, or they can be more dispersed areas in which single males display. In both situations, females will survey a number of males and will then mate with one or a few of them. The result is a minority of males copulate with most of the females that visit the lek. The satin bowerbird provides one example of a lekking species. The delayed maturation of male bowerbirds and the sex difference in developmental activities (i.e., bower building) are likely the evolutionary result of male-male competition and female choice, although the relative contribution of these different components of sexual selection are not known. Whatever the relative contribution, female choice has contributed to the evolution of sex differences in the life history of bowerbirds. Peacocks (Pavo cristatus) provide another example of a lekking species but one in which male-male competition is minimal and female choice largely determines which males reproduce and which do not (Htglund & Alatalo, 1995; Petrie, 1994; Petrie, Halliday, & Sanders, 1991). Unlike peahens, peacocks develop large tail trains with varying numbers of eyespots. Males display unfolded trains to females, and females choose mates on the basis of the length of the train and the number of eyespots (Petrie et al., 1991). Train length and number of eyespots are reliable predictors of the growth and survival rate of the males' offspring and are thus honest (i.e., cannot be faked; Zahavi, 1975) indicators of the genetic quality of the male (Petrie, 1994). As with bowerbirds, sex differences in life history traits are found. Peahens begin to reproduce at 2 years of age, whereas males do not develop their full trains until 3 years of age and do not establish a lekking display site until 4 years

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of age (Manning, 1989; Petrie, personsal communication, August 28,2001). Some males successfully mate at age 4, whereas other males do not mate until later years and some not at all. In this species, the development of the sexually selected traits that females use in mate choice decisions has resulted in accompanying changes in the life history of males, including a lengthening of the developmental period and an increase in size at maturity (for related discussion see Brooks & Kemp, 2001; Wiley, 1974). C. PHENOTYPIC PLASTICITY

Phenotypic plasticity is a common feature of life history traits. That is, the ontogenetic expression of life history traits is influenced by ecological conditions within the constraints of reaction norms (Stearns, 1992). Plasticity of life history traits is often conceived in terms of nonsocial factors, such as predation, food availability, or rainfall (e.g., Stearns & Koella, 1986; Sultan, 2000). Life history traits that have been shaped by sexual selection are also likely to show phenotypic plasticity, but the ontogenetic expression of these traits should be more strongly influenced by social competition and dynamics than by nonsocial conditions (Rohwer et al., 1980; Selander, 1965). In fact, the relation between many ecological variables, such as food availability, and the plastic expression of life history traits may be moderated by social competition. The age of reproductive maturity in many species is influenced by access to high-quality foods (Stearns & Koella, 1986), which in turn is often influenced by social competition. In many species of primate, coalitions of related females compete for access to high-quality food sources, such as fruit trees (Wrangham, 1980). Socially dominant coalitions gain access to these foods, and the combination of dominance and better nutrition is related to a number of indices of reproductive maturity and success, including age at first conception and number of offspring surviving to maturity (Silk, 1993). The dynamics of sexual selection also appear to influence the ontogenetic expression of many life history traits. As with bowerbirds, males are often physiologically able to reproduce many years before they actually reproduce (Wiley, 1974). The reproductive delay can be due to competition with older and more dominant males or a female preference for older males (Brooks & Kemp, 2001; Selander, 1965). In many lekking species, for instance, males must establish and defend a display territory, and older males typically have an advantage over younger males in competition for these sites (Wiley, 1974). In some species, the physiological stress associated with male-male competition and the behavioral subordination to more dominant males, as well as other forms of social competition, can delay physiological maturation or reduce reproductive potential (e.g., reduce size of testes) in adult males (Dixson, Bossi, & Wickings, 1993; Walter & Dittami, 1997; Waiters & Seyfarth, 1987). As noted earlier, testicular maturation in male baboons is related to the social rank of their mother (Alberts & Altmann, 1995).

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In short, the presence of dominant males often results in the inhibition of competitive behavior in younger males and can delay the onset of reproductive maturity and inhibit the expression of traits associated with female choice. Note also that nonsocial influences on the expression of life history traits, such as food availability in the absence of social competition, can strongly influence the later ability to compete for mates or attract mating partners (e.g., Nowicki, Peters, & Podos, 1998).

V. Human Developmental Sex Differences A comprehensive understanding of human developmental sex differences can only be achieved through consideration of the evolution and proximate functions of life history traits (Bjorklund & Pellegrini, 2002; Bogin, 1999; Geary & Bjorklund, 2000; Hill & Kaplan, 1999; Kaplan, Hill, Lancaster, & Hurtado, 2000; Kenrick & Luce, 2000; Lancaster & Lancaster, 1987). Current models of human life history focus on the complexity of the foraging demands in traditional societies (Kaplan et al., 2000) or on social competition (Alexander, 1987, 1989; Geary & Flinn, 2001). In fact, both forms of selective pressure are likely to have been important. In many traditional societies, men provide--through hunting--the majority of calories and protein consumed by their social group (Ember, 1978; Kaplan et al., 2000). The acquisition of hunting skills requires many years of practice and experience, which has been interpreted as a selective pressure for an increase in the length of the developmental period during which hunting (and foraging for females) skills are practiced (Kaplan et al., 2000). The proceeds of hunts are also related to male-male competition (social status) and female choice of mating partners (Hill & Hurtado, 1996), components of sexual selection. Moreover, the extraordinary hunting and foraging skills of humans in traditional societies, and presumably during human evolution, contribute to the ecological dominance of most human groups, which in turn significantly alters the pattern of selective pressure, as noted by Alexander (1989, p. 458): The ecological dominance of evolving humans diminishedthe effects of "extrinsic" forces of natural selection such that within-speciescompetitionbecame the principle "hostileforce of nature" guidingthe long-termevolutionof behavioralcapacities,traits, and tendencies. With the achievement of ecological dominance, natural selection becomes a struggle with other human beings for access to and control of the social (e.g., competition for mates), biological (e.g., food), and physical (e.g., territory) resources that covary with survival and reproductive outcomes (Geary, 1998). Elaborating on Alexander (1989), Geary and Flinn (2001) argued that the resulting social competition was the primary selective pressure driving the coevolution of a suite of

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human characteristics, including several life history traits described in Table II. In this view, the hunting and foraging demands described by Kaplan et al. (2000; Hill & Kaplan, 1999) are intertwined within a broader suite of social-competitive selection pressures, including intrasexual competition and intersexual choice. My goal in the remainder of this chapter is to extend the social competition model to provide a theoretical frame for understanding sex differences in human life history and for organizing psychological research on sex differences in developmental activities, such as play patterns; implications for understanding sex differences in human cognition and cognitive development are described elsewhere (Geary, 1998, 2002). In the first two parts, I provide sketches of sexual selection in humans and sex differences in life history traits, respectively. In the third part, I describe the framework for and research on developmental activities. The behavioral and social activities described in all parts are more strongly influenced by inherent and implicit processes than by conscious choice. In other words, as with other species, children and adults are inherently biased to engage in activities that covaried with survival and reproductive outcomes during human evolution, whether or not they are consciously aware of the proximate function (e.g., to attract mates) of the activities (Geary, 1998).

TABLE II Unique and Unusual Traits Related to Social Competition and Human Life History

L Large brain and complex social competencies 1. The human neocortex is 35-60% larger than expected for a primate of the same overall body and brain size (Rilling & Insel, 1999). 2. The neocortex apparently is larger than that of other primates in those areas that support social competencies that are unique to humans (Rilling & Insel, 1999), that is, theory of mind (Baron-Cohen, 1995) and language (Pinker, 1994). II. High levels of paternal investment 1. Paternal investment is only evident in 3-5% of mammalian species (Clutton-Brock, 1989). 2. Even for these species, humans are unique in that paternal investment occurs in a social context of large multimale-multifemale communities and where most adult members of these communities reproduce (Alexander, 1990; Geary, 2000). IlL Long developmental period and adult life span 1. Relative to other mammals and primates, children have a very long developmental period characterized by slow development during middle childhood and high dependency on adult caregiving (Bogin, 1999). 2. Relative to other great apes, humans have a very long adult life span, and low juvenile and adult mortality rates (Allman et al., 1993; Hill et al., 2001). IV. Menopause 1. Menopause may enable women to heavily invest in their later born children or in grandchildren (Hawkes et al., 1998; Williams, 1957). Adapted from Geary and Flinn (2001, p.6).

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A. SEXUALSELECTIONDURINGHUMANEVOLUTION Unlike most other mammals, both women and men invest in parenting, although men on averge do not invest as much as women (Geary, 2000). In natural environments, paternal investment supports the intensive parenting and long developmental period that is characteristic of humans and appears to allow parents to successfully raise more children than would otherwise be the case (see Table II; Geary & Flinn, 2001; Lancaster & Lancaster, 1987; Kaplan et al., 2000). In addition, paternal investment complicates the dynamics of sexual selection, resulting in male choice and female-female competition, on top of the standard components of male-male competition and female choice. Intensive parenting and the complexity of sexual selection create six interrelated classes of social relationship that involve conflict and competition, as described in Table III. These forms of social relationship capture the within-species competition noted by Alexander (1989) and are likely to have been (and continue to be) important forces in human cognitive, brain, and social evolution, as well as potent influences on evolutionary change in human life history and developmental activity. Extended spousal and parent-child relationships create conditions that favor the evolution of complex sociocognitive competencies, as both forms of relationship involve not only cooperation but also manipulation and deception as related to attempts to gain access to resources that covary with survival and reproductive outcomes (e.g., Geary, 2000; Trivers, 1974). Further discussion of these relationships is beyond the scope of this article, but here I summarize work on intrasexual competition and intersexual choice. 1. Intrasexual Competition

On the basis of the fossil record, males apparently were larger than females throughout hominid evolution, suggesting physical male-male competition and a sex difference in parental investment (e.g., McHenry, 1991). The fossil record does not provide insights into the intensity or form of this male-male competition (Plavcan & van Schaik, 1997b), but patterns throughout human history and in extant cultures suggest competition involving male coalitions and one-on-one competition for status and dominance within coalitions (Chagnon, 1988; Geary, 1998; Geary & Flinn, 2002; Horowitz, 2001; Wrangham, 1999). To illustrate, for forest dwelling Ache (hunter-gatherer society, Paraguay) coalitional warfare with non-Ache accounted for 36% of all adult male deaths, and an additional 8% of men died during status-oriented club fights with other Ache men (Hill & Hurtado, 1996). This pattern of coalitional and within-coalition male-male competition is common in traditional societies (Keeley, 1996) and has evolutionarily significant survival and reproductive consequences. Dominant men in dominant coalitions typically have more wives and more surviving children than do other men (e.g., Chagnon, 1988). In addition to facilitating male-male competition, men's coalitions also enable them to achieve ecological dominance, that is, to efficiently extract resources

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TABLE III Forms of Social Conflict and Competition Intrasexual competition L Male-male competition 1. In traditional societies, men form coalitions that compete for control of mating dynamics (e.g., exchange of brides) and for control of the resources that covary with survival and reproductive outcomes in the local ecology (Chagnon, 1988; Geary, 1998). Men also form dominance hierarchies within the in-group coalitions and compete for position (and thus influence) in the hierarchy. Competition is often physical and deadly (Keeley, 1996).

II. Female-female competition 1. Women compete for access to resources, including access to resource-holding or socially influential men. Relative to men, this competition is less physical (Campbell, 1999) and involves subtle manipulation of social relationships, with the goal of organizing these relationships to maximize the woman's access to resources that covary with survival and reproductive outcomes in the local ecology (Geary, 2002). Intersexual choice I. Male choice 1. Paternal investment leads to the prediction that men will be selective in their mate choices (Trivers, 1972), and this is the case. Men's mate choices are influenced by fertility cues (e.g., age) as well as by indicators of women's social and maternal competence (Geary, 1998).

II. Female choice 1. Women's mate choices are influenced by men's social and parental competence. More so than men, women also focus on men's social status, including material resources, social influence, and cues to their ability to acquire and maintain these resources (Buss, 1989, 1994). Women are also sensitive to their ability to influence potential mates and thus gain access to their resources. Family conflict L Spousal 1. Spouses, of course, cooperate in raising children, but extended maternal and paternal investment also results in strong potential for conflicts of interest (Kaplan et al., 2000; Svensson & Sheldon, 1998). Conflicts are predicted to center on (1) extent of maternal versus paternal investment, (2) resource control (e.g., spending on children or status-oriented objects), and (3) marital fidelity.

IL Parent-offspring and sibling 1. Across species, conflicts of interest are endemic to parent-offspring relationships (Trivers, 1974). Parents, of course, invest time and resources to promote the well-being of offspring, but offspring often press for additional resources, sometimes with accompanying morbidity and mortality costs to parents (Westendorp & Kirkwood, 1998). The long developmental period of humans results in an extended parent-child relationship and thus the potential for extended conflicts over parental allocation of resources. In the context of these relationships, children are predicted to attempt to secure from parents (and to a lesser degree kin) resources that facilitate (1) growth and maintenance and (2) reproductive potential. 2. Siblings will also compete for parental resources.

from the environment through hunting and to maintain a comparatively risk-free (e.g., low predation risk) territory for their social group (Tiger, 1969). These male coalitions consist of related men and define the basic social structure of the group. Women tend to emigrate into the group of their husband, although in many societies they maintain ties to their kin (Pasternak et al., 1997; Seielstad,

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Minch, & Cavalli-Sforza, 1998). In any case, male coalitions provide protection from other coalitions, and provision their wives and children, although women also provide food through gathering (Ember, 1978; Geary & Flinn, 2001; Kaplan et al., 2000). Within these groups, women form a small network of relationships with other women and provide social and emotional support to one another (Taylor et al., 2000). Women also work to organize social relationships within the wider group to divert additional resources to themselves and to their children (Geary, 2002; Geary & Flinn, 2002). One result is conflicts of interest with other women. The resulting female-female competition focuses on disrupting the social network of their female competitors (often co-wives), as well as competing for the attention and resources of men within the group. This form of interpersonal competition has been termed relational aggression by Crick and her colleagues (Crick, Casas, & Mosher, 1997). The function of male-male (achieving status and cultural success; Irons, 1979) and female-female (control of interpersonal relationships; Geary, 2002) competition is the same from one society or historical period to the next: to gain access to and control of the resources that covary with survival and reproductive outcomes in the local group and ecology. However, the form of intrasexual competition shows considerable phenotypic plasticity, varying with cultural mores (e.g., prohibitions against polygyny) and social conditions, such as the operational sex ratio (Flinn & Low, 1986; Geary, 1998, 1999; Low, 1989). In some societies, men achieve cultural success by killing other men, whereas in other societies they achieve cultural success by obtaining an education and securing a high-income job. The underlying motive is always the same, however: to achieve dominance over other men and thus increase access to culturally important resources and, through this, reproductive opportunity (P6russe, 1993). Patterns of intrasexual choice also show phenotypic plasticity, as illustrated by cross-cultural variability in women's preference for husbands who have or have not killed other men (preference varies with whether or not killing confers social status; Geary, 1998). 2. Intersexual Choice

Patterns of intersexual choice can influence the form and intensity of intrasexual competition. As an example, men's focus on physical attractiveness (e.g., fertility cues) in choosing a spouse intensifies female-female competition in this area, with women competing by highlighting or manipulating (e.g., through makeup or padded bras) these cues (Buss & Shackelford, 1997). Similarly, women's preference for culturally successful and resource-holding men intensifies male-male competition for control of culturally important resources, especially in cultures where female choice is not suppressed (Geary, 1998). The reproductive preferences of men and women conflict directly in other ways (Buss & Schmitt, 1993). As with other mammalian males, men can reproduce without paying the cost of parental investment, but women do not have this option. The result is men, on average, are more interested in casual sexual relationships than are women and

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will sometimes manipulate women into sexual relationships and then abandon them if a pregnancy occurs (Oliver & Hyde, 1993; Symons, 1979). Men face a different issue. When they invest in children, the paternity of these children is never certain. Although firm estimates are not yet available, perhaps 10-15% of children are sired by a man who is not the women's husband and putative father (Geary, 2000). In these situations, the man is being manipulated by his spouse into raising the children of another man. B. SEX DIFFERENCES IN LIFE HISTORY

A relation between heritable variability in life history traits (e.g., parenting) and survival and reproductive outcomes in human populations has been inferred in many analyses (Geary, 2000; Kaplan et al., 2000), but, unfortunately, rarely evaluated in genetic studies of reproductive fitness. In one such twin study, the relation between lifetime reproductive fitness (number of surviving children) and three life history traits--age at menarche, at first reproduction, and at menopause-was assessed for Australian women over the age of 45 (Kirk et al., 2001). Age at first reproduction (early 20s vs late 20s) was significantly related to reproductive fitness (controlling for educational level and religious affiliation), with earlier reproduction resulting in more children during the reproductive life span. The covariation between age of first reproduction and reproductive fitness was due, in part, to shared genetic influences, indicating that a subset of genes influences both age of first reproduction and lifetime reproductive fitness in the Western women assessed in this study. Similar studies have not yet been conducted for all of the life history traits described in Table II, although individual differences in many of these traits (e.g., life span) have been shown to be related to both heritable and environmental factors (e.g., Herskind et al., 1996). In any event, these traits suggest that selection favored a life history pattern whereby humans invested heavily in a small number of offspring, presumably so that these offspring acquire sophisticated social, behavioral, and cognitive skills, that is, acquired reproductive potential. An unresolved issue is the selection pressures that contributed to the evolution of this suite of life history traits. As stated, theoretical models tend to focus on ecological (e.g., food acquisition, predation) pressures, social pressures (e.g., between group competition), or some combination. To the extent that social competition contributed to the evolution of human life history, sex differences in the associated traits should vary in ways consistent with sexual selection and other forms of social competition. In Table IV, I present a series of predictions regarding how sexual selection might have influenced the evolution of sex differences in human life history. Many of the empirical studies cited therein and described in the following sections are consistent with the predictions, but definitive tests will require the type of analysis conducted by Kirk et al. (2001). Before proceeding to the discussion of sex differences, I provide a general outline of human life history in the first part.

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TABLE IV Predicted Influence of Sexual Selection on Human Life History Women's life history L Female-female competition 1. Female-female competition is highly social, involving the manipulation and disruption of the relationships of other girls and women as related to mating dynamics and resource control (Crick et al., 1997). Based on the relations among social competition, length of the developmental period, and brain size in primates (Joffe, 1997), human female-female competition is predicted to favor delayed maturation and developmental activities that involve the practice and refinement of competition-related social competencies. II. Male choice 1. Due to menopause and women's declining fertility beginning in the late 20s (Menken et al., 1986), men base mate choice decisions, in part, on physical indicators of fertility. These indicators, such as large eyes, are correlated (across species) with youth. Male choice is thus predicted to favor earlier maturation in women and thus retainment of these cues, although selection for earlier maturation is balanced by the conflicting benefits of larger size and thus delayed maturation (Kirk et al., 2001; Stearns & Koella, 1986).

Men's life history L Male-male competition 1. Men are predicted to have and evidence the same life history pattern found in other mammalian species with physical male-male competition. Included among these traits are, in comparison to women, a shorter life span, slower growth and longer developmental period, larger adult size, higher mortality rate at all ages, and higher levels of risk taking and intrasexual violence (e.g., Allman et al., 1998; Wilkins, 1996; Wilson & Daly, 1985). 2. In relation to other mammals, the previously noted pattern has likely been mitigated by paternal investment, resulting in an evolutionary reduction in the magnitude of the sex differences in many life history traits (e.g., maximum life span). 3. Coalitional male-male competition is necessarily a complex and highly social activity (Geary & Flinn, 2002). As with women, social competition should favor delayed maturation and developmental activities that involve the practice and refinement of associated competencies. II. Female choice 1. Female choice is related, in part, to the physical and social cues associated with male-male competition and should thus intensify these aspects of male-male competition, such as the tendency toward behavioral risk taking (Buss, 1989; Kelly & Dunbar, 2001). 2. Female choice is also related to physical indicators of health (e.g., Gangestad, Bennett, & Thornhill, 2001), and the expression of these indicators may be costly for many men (Shackelford & Larsen, 1997). For instance, the androgens that result in the development of physical traits (e.g., masculine jaw) associated with female choice are predicted to compromise the immune system (Folstad & Karter, 1992) and thus the physical health and development of some males (Geary, 1998).

1. Pattern of Human Life History The focus of human life history research conducted by biologists and anthropologists is largely on the growth component of somatic effort shown in Figure 1 as well as reproductive correlates. These studies reveal that in traditional societies and in preindustrial Europe, a typical life history pattern involves women achieving menarche at about 15-16 years of age, marrying soon thereafter, and having their first child between 18 and 22 years of age (Kaplan et al., 2000; Korpelainen, 2000;

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Stearns & Koella, 1986). For women, peak fertility is achieved in the mid- to late 20s and gradually declines to near zero by age 45 (Menken, Trussell, & Larsen, 1986; Wood, 1994), resulting in a 25-year reproductive span. During this span, a common interbirth interval is 2-4 years and a common pattern is for women to have four to six children before the onset of menopause, although one to three of these children do not survive to adulthood (Blurton Jones, 1986; Hill & Kaplan, 1999; Lummaa, 2001). Women and men who survive to age 15 years will, on average, live to their mid-50s (e.g., Hill & Hurtado, 1996). In many traditional societies, socially dominant men will have several wives, but often do not marry their first wife until their 20s or later; many other men will never marry (e.g., Borgerhoff Mulder, 1990; Chagnon, 1988; Murdock, 1949). Relative to current industrial society, a 30-40% mortality rate before the age of 15 and an average life span of 55 for those who survive to adulthood seems rather dismal. However, in comparison to other apes and mammals, the human pattern represents comparatively low infant and child mortality risks (Lancaster & Lancaster, 1987) and a comparatively large number of children surviving to adulthood relative to other apes (Hill et al., 2001). The achievement of comparatively low infant and child mortality risks and a comparatively large number of surviving children is not likely to be achievable without men's parental investment, at least in traditional societies, whether the tendency to provide this investment evolved primarily for the provisioning of women and dependent children (Kaplan et al., 2000) or to facilitate a broader suite of sociocompetitive competencies (Alexander, 1989; Geary, 2000; Geary & Flinn, 2001; but see Hawkes, O'Connell, Blurton Jones, Alvarez, & Charnov, 1998). As with other species, both heritable and environmental factors appear to contribute to individual differences in human life history traits (de Bruin et al., 2001; Kirk et al., 2001), and apparently within the constraints of norms of reaction (Stearns & Koella, 1986). As an example, Kirk et al. found that individual differences in women's life history traits were influenced, in part, by genetics, with heritability estimates ranging from .51 for age of menarche to .21 for age at first reproduction. Stearns and Koella demonstrated that well nourished and healthy girls achieve menarche earlier than other girls, indicating an important environmental component to the expression of this life history trait. At the same time, there are genetic constraints such that age of menarche is not typically achieved earlier than age 11 years, with a mean of 13 years, even in well-nourished and healthy populations (Kirk et al., 2001). 2. Sexual Selection and Women's Life History Whatever the primary selective advantage, paternal investment necessarily coevolved with women's life history traits. The trade-offs associated with paternal investment are female-female competition over this investment and male choice (Trivers, 1972), along with the compromises needed to maintain a long-term

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spousal relationship. As noted earlier and in Table IV, social competition covaries with a longer developmental period and a larger brain, at least in primates (e.g., Joffe, 1997). The longer developmental period presumably allows juveniles to practice and refine sociocompetitive competencies, that is, to improve their reproductive potential. If so, then female-female competition would have contributed to the long juvenile period found in humans and girls' developmental activities should involve, in part, a preparation for this competition in adulthood as well as preparation for family life as a parent and spouse, as described later. Across societies, male choice is related to physical indicators of female fertility as well as to personal and social indicators of fidelity and thus certainty of paternity (Buss, 1994; Buss, Larsen, Westen, 1996; Geary, 1998). Physical indicators of fertility include age, the hip-to-waist ratio, breast symmetry, and a youthful appearance (Cunningham, 1986; Kenrick & Keefe, 1992; Mr Soler, & Thomhill, 1995; Singh, 1993). The hip-to-waist ratio is a natural consequence of birthing children with large brains, but also influences male choice and may have been exaggerated as a result (Singh, 1993). Enlarged breasts in the absence of lactation is unusual in mammals and may have been shaped by male choice. Moreover, men's rating of women's physical attractiveness is related to breast symmetry, that is, similarity in the size of the two breasts. Breast symmetry, in tum, is a reliable indicator of women's fertility (Mc~ller et aL, 1995). A focus on youthful appearance follows from the age-related decline in women's fertility and appears to have influenced the evolution of certain facial characteristics in women. These characteristics are cross-species indicators of youth, including relatively large eyes and a small chin (Cunningham, 1986). In short, male choice may have been influenced by the life history pattem of women (e.g., menopause) and may have influenced the evolution of certain aspects of women's physical development. As noted in Table V, phenotypic plasticity in some aspects of women's life history is related to social conditions and to female hormones (Ellis, McFadyenKetchum, Dodge, Pettit, & Bates, 1999; Flinn & Low, 1986; Wilson & Daly, 1997). For example, girls with a warm relationship with their father and a father who is highly invested in the family experience menarche later than do gifts living in father-absent homes or with an emotionally distant father (Ellis et al., 1999). Relationship with father may be a proximate indicator of the stability and warmth of the girls' later spousal relationships as well as an indicator of mortality risks in adulthood (a deceased father cannot be invested or warm; Belsky, Steinberg, & Draper, 1991; Chisholm, 1993; Draper & Harpending, 1988). These cues in turn are presumed to influence women's reproductive pattem in adulthood, such that paternal warmth is associated with delayed maturation. Delayed maturation, in turn, should enable the acquisition of additional sociocompetitive competencies and greater reproductive potential. The reproductive potential may involve the acquisition of social and emotional traits that support high cooperation with a spouse and high investment in a small number of children (MacDonald, 1992).

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TABLE V Phenotypic Plasticity in Human Life History L Family functioning and wealth 1. Family functioning can influence the phenotypic expression of some life history traits in both men and women. As examples, psychosocial stressors during childhood are associated with smaller adult size and lower testosterone levels in adulthood for men (e.g., Flinn et al., 1996) and a warm relationship with father is associated with later maturation for women (Ellis et al., 1999). 2. Wealth of the family and kin group can influence nutritional and physical health during development and result in an earlier age of maturation, larger adult size in women and men, and lower infant and child mortality risks (Herman-Giddens, Wang, & Koch, 2001; Korpelainen, 2000; Stearns & Koella, 1986). II. Culture and the operational sex ratio 1. In ecologies with high adult mortality (often due to male-on-male violence) women and men have an earlier age of reproduction and have more children during the reproductive life span (Chisholm, 1993; Wilson & Daly, 1997). In ecologies with high infant and child mortality risks, women and men have more children during their reproductive life span (Korpelainen, 2000). 2. Socially imposed monogamy, as in Western culture, moderates the dynamics of intersexual choice and intrasexual competition. The intensity of male-male competition decreases and the standards of male choice increase, whereas the intensity of female-female competition increases but female choice does not appear to be as strongly affected (Geary, 1998). As an example, in these societies men's mate choices are influenced by women's material resources (e.g., dowry, income; Gaulin & Boster, 1990). As a result, the marriage and reproduction of women may be delayed while these resources are accrued by women and/or their families. 3. The intensity of competition for cultural resources and social status is predicted to result in corresponding delays in men's reproductive opportunities beyond the age of physical maturation, as younger men will typically be disadvantaged in comparison to older men, both in terms of female choice and male-male competition (e.g., Borgerhoff Mulder, 1988). One possible result is age-related difference in men's reproductive strategies, with younger men engaging in more opportunistic mating and older men investing in children (Draper & Harpending, 1988). 4. The operational sex ratio moderates the dynamics of intersexual choice and intrasexual competition. When the number of marriage-age men is lower than marriage-age women, men have more influence (male choice) over mating dynamics and the intensity of female-female competition increases. One result is that men invest more in mating effort and less in parenting (Geary, 2000; Guttentag & Secord, 1983). III. Sex hormones and endocrine functions 1. As with other species, hormonal and endocrine mechanisms (e.g., testosterone) are predicted to moderate and perhaps mediate many sex differences in life history traits and aspects of phenotypic plasticity (Sinervo & Svensson, 1998). Hormones associated with developing the physical and social competencies associated with male-male competition likely result in the trade-offs of higher mortality rates and a shorter maximum life span. Female hormones may facilitate immune responses in women, but with the trade-off of higher risk of autoimmune disorders (Wizemann & Pardue, 2001). 2. Hormonal fluctuations across the menstrual cycle appear to influence women's mate choice preferences, the likelihood of cuckolding her mate, and patterns of relationship jealousy; they may also influence indicators of fertility associated with male choice (Bellis & Baker, 1990; Gangestad & Thornhill, 1998; Geary, DeSoto, Hoard, Sheldon, & Cooper, 2001; Scutt & Manning, 1996).

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Other evidence of phenotypic plasticity in women's life history comes from the work of Wilson and Daly (1997), who found that age of first reproduction, number of children borne per woman, mortality risks, and local resource availability are all interrelated in modern-day Chicago. Women who grow up in contexts with high risk of violent death (related to male-male competition over limited resources) begin reproducing at an earlier age and have more children during their reproductive life span than do women who grow up in low-risk, high-resource ecologies (see also Chisholm, 1993). Similarly, in preindustrial Europe, women living in contexts with high infant and child mortality risks had more children during their reproductive life span than did women living in contexts with lower mortality risks (Korpelainen, 2000). Wider social conditions also appear to influence women's reproductive pattern. In societies in which men's reproductive options are restricted by socially imposed monogamy, women's financial contributions to the family (e.g., a dowry) become a more prominent feature of male choice than in other societies (Flinn & Low, 1986; Geary, 1998). In these contexts, age of first marriage and reproduction may be delayed while women or her kin accrue the resources needed (e.g., for the dowry) to attract a high-status spouse (Gaulin & Boster, 1990). Hormones also influence women's reproductive and life history traits. For instance, hormonal fluctuations across the menstrual cycle are correlated with women's mate choice preferences, such that women may be more likely to cuckold their partner during the time of rising fertility risk, that is, around the time of ovulation (e.g., Bellis & Baker, 1990; Gangestad & Thornhill, 1998). Hormones may also contribute to the longer life span of women relative to men. Female hormones may contribute to enhanced immune responses and reduced risks for certain forms of premature death (e.g., heart disease; Wizemann & Pardue, 2001). All these findings are in keeping with the earlier described patterns of phenotypic plasticity in other species. For instance, Daly and Wilson's (1997) intriguing findings are consistent with the cross-species pattern of earlier maturation being related to high adult mortality risks and phenotypic plasticity in age of first reproduction (e.g. Roff, 1992; Reznick & Endler, 1982). However, all of the human studies are based on phenotypic correlations and thus the patterns might reflect phenotypic plasticity, genetic correlations (e.g., between age of menarche and paternal investment), or, most likely, some combination. 3. Sexual Selection and Men's Life History

With the exception of paternal investment, the pattern of men's life history is the same as that found for other mammalian species in which males compete physically for access to females or for control of the resources females need to reproduce (see Table V; Allman et al., 1998; Leigh, 1996; Wilson & Daly, 1985). Even paternal investment does not alter the general pattern, because men invest

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less in parenting and more in mating than do women. Relative to females, the life history traits that men share with other mammalian males include an average and maximum life span that is shorter than females' (Allman et al., 1998), a slower growth rate and longer growth spurt (Leigh, 1996; Tanner, 1990), higher mortality rates at all ages (Wizemann & Pardue, 2001), and greater behavioral risk taking and higher rates of competition-related violent death in adolescence and young adulthood (Daly & Wilson, 1988; Wilson & Daly, 1985). As with other species, all these life history traits are influenced by testosterone and other hormones and appear to involve a preparation for or engagement in male-male competition (e.g., Folstad & Karter, 1992; Dabbs & Dabbs, 2000; Wizemann & Pardue, 2001). Many of these same physical (e.g., height) and behavioral traits (e.g., risk taking) also influence female choice (Beck, Ward-Hull, & McClear, 1976; Kelly & Dunbar, 2001). The slow growth and extended developmental period result in larger men than women (Tanner, 1990, 1992). Among other physical and physiological sex differences that become exaggerated during this time are running speed, physical strength, and physical activity level, as well as throwing distance, velocity, and accuracy (Eaton & Enns, 1986; Kolakowski & Malina, 1974; Thomas & French, 1985). These and related sex differences are often attributed to the division of labor in general and men's hunting in particular (Kolakowski & Malina, 1974; Tiger, 1969). However, the finding that men are consistently better on tasks that involve the ability to track thrown objects and to evade or block these objects (Watson & Kimura, 1991) may be the evolutionary result of male-male competition that involved the use of projectile (e.g., rocks) and blunt force weapons (see Keeley, 1996, for examples), with tracking and blocking being evolved defensive competencies needed to avoid projectiles (Geary, 1998). As with women, individual differences in these physical and life history traits appear to be influenced by genetic and environmental factors (Gilger, Geary, & Eisele, 1991; Martorell, Rivera, Kaplowitz, & Pollitt, 1992). Due to slower growth, greater physical activity levels, and perhaps the immunosuppressive effects of testosterone, boys are more severely affected--physically and cognitively--by poor early conditions than are girls (Flinn, Quinlan, Decker, Turner, & England, 1996; Martorell et al., 1992). One result is that the sex difference in physical size is smaller in poorly nourished populations (Gaulin & Boster, 1992). The form of male-male competition clearly shows phenotypic plasticity, as noted in Table V. In contexts with high mortality risks, often due to male-on-male violence, men reproduce sooner and often show lower levels of parental investment than do men in other contexts (Geary, 2000; Wilson & Daly, 1997). When physical male-onmale violence is suppressed and the attainment of culturally important resources (e.g., money) requires prolonged education and training, men have lower mortality risks and a longer life span, but they reproduce later and often have fewer children (Geary, 1998; Ptrusse, 1993). To varying degrees, female choice is important in

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all these contexts. In the latter contexts, for instance, women's preference for culturally successful men results in delayed marriage and reproduction for many men (Buss, 1996). As with other species, the proximate influences on men's (and women's) life history traits almost certainly include sex hormones and other endocrine functions as well as social context. The hormonal correlates of male-male competition are highly sensitive to social and contextual cues (Geary & Flinn, 2002). Loss in male-male competition often results in heightened elevation of stress hormones, reduced testosterone levels, and perhaps compromised health (Dabbs & Dabbs, 2000). For men, early childhood stressors, such as extended family conflict or lower levels of paternal investment, are associated with atypical stress responses later in life and smaller adult size (Flinn et al., 1996). These, in turn, can result in disadvantages with respect to both male-male competition and female choice. Still, many questions regarding men's life history and phenotypic plasticity in the expression of these traits remain unanswered. For instance, is the relation between adult mortality risks and early reproduction an expression of phenotypic plasticity, of genes that influence both early reproduction and tendency toward male-onmale violence, or of some combination? Do the immunosuppressive effects of stress hormones affect some men more than others, as appears to be the case with other species (Folstad & Karter, 1992)? Do these effects vary at different points in the life span? C. SEX DIFFERENCES IN DEVELOPMENTAL ACTIVITY

Unlike biologists and anthropologists, the research focus of developmental psychologists is largely on the developmental activity component of somatic effort shown in Figure 1. The details of developmental activity as related to somatic effort and the accumulation of reproductive potential are shown in Figure 6. In this view, parent-child relationships, the wider kin network in traditional societies, and

Developmental Activity Growth and Maintenance Parent-Offspring Relationship

Kin

Relationships

Self-Initiated Activities

Reproductive Potential Parent-Offspring Relationship

Peer Relations and Social Play

Solitary Play

Fig. 6. Components of developmental activity. Parent-offspring relations emerge from parents' reproductive efforts and the efforts of offspring to obtain additional parental resources, as related to the offspring's somatic effort and the accumulation of reproductive potential

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some self-initiated activities during childhood and adolescence, such as foraging in traditional societies, function to promote growth and maintenance during specific developmental periods (Bjorklund, 1997; Bjorklund & Pellegrini, 2002; Geary & Bjorklund, 2000). The function of other activities is, in theory, to accumulate reproductive potential, that is, to acquire the physical, social, behavioral, and cognitive competencies that enable successful reproduction in adulthood, whether the reproductive effort involves mating, parenting, or some combination. The function of parent-offspring relationships as related to social competition and reproductive effort (e.g., acquiring resources that facilitate competition with peers) in adulthood is described elsewhere (Geary & Flinn, 2001). The focus here is on the developmental activity component of peer relationships and various forms of play. As stated previously, from a life history perspective one function of these activities is to refine the social, behavioral, and cognitive competencies that covaried with survival and especially~given comparatively low adult mortality rates~reproductive outcomes during the species' evolutionary history (Geary, 1998; K~i~ & Jokela, 1998; Kaplan et al., 2000; Mayr, 1974). The basic skeletal structure of these competencies appears to be inherent, but fleshed out and adapted to local conditions as the juvenile engages in the associated activities, such as play hunting (Gelman, 1990; Gelman & Williams, 1998). The most fundamental of these competencies coalesce around the domains of folk psychology, folk biology, and folk physics (e.g., Atran, 1998; Leslie, 1987; Mandler, 1992), in keeping with the position that humans are fundamentally motivated to gain access to and control of the social (e.g., mates), biological (food), and physical (e.g., territory) resources that covary with survival and reproductive outcomes in the local social group and ecology (Geary, 1998; Geary & Huffman, 2002). The general prediction is that children will evince a pattern of self-initiated activities that results in the practice and refinement of social, behavioral, and cognitive competencies that covaried with survival and reproductive outcomes during human evolution. The specific predictions relating to the current discussion are: (1) sex differences will emerge in the form of intrasexual relationships and these will be related to patterns of male-male and female-female competition; (2) girls will engage in more play parenting and family-oriented play, on the basis of the sex difference in parental investment; and (3) boys will engage in more activities associated with ecological dominance, specifically activities involved in hunting and territory maintenance. Full review and discussion of research related to these predictions is beyond the scope of this chapter (see Geary, 1998, 2002), but overviews are provided in the following sections. 1. Peer Relationships and Social Play Basic patterns of children's peer relationships and social play are considered in the following subsections, including social segregation and separate looks at prominent features of the social activities of boys and gifts.

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a. Social Segregation. If males and females differed in the competencies that covaried with survival and reproductive outcomes during human evolution, then children are predicted to segregate by sex and engage in activities that mirror this evolutionary history (Geary, 1998). In keeping with the first prediction, one of the most consistently found features of children's social behavior is the formation of same-sex play and social groups (Maccoby, 1998). Such groups are evident by the time children are 3-years-old and become increasingly frequent throughout childhood. For example, in a longitudinal study of children in the United States, 4- to 5-year-olds spent 3 h playing with same-sex peers for every 1 h they spent playing in mixed-sex groups; as 6- to 7-years-olds, the ratio of time spent in samesex versus mixed-sex groups was 11:1 (Maccoby & Jacklin, 1987). The same pattem has been found for children in Canada, England, Hungary, Kenya, and Mexico (Strayer & Santos, 1996; Turner & Gervai, 1995; Whiting & Edwards, 1988). The degree of segregation varies across contexts and is most common in situations in which children are free to form their own social groups (Maccoby, 1988; Strayer & Santos, 1996). The proximate mechanisms driving child-initiated segregation include the different play styles of girls and boys (described later) and differences in the strategies used to attempt to gain control of desired resources (e.g., toys) or to influence group activities. In situations where access to a desired object is limited, boys and girls use different social strategies (Charlesworth & Dzur, 1987). More often than not, boys gain access by playfully shoving and pushing other boys out of the way, whereas girls gain access by means of verbal persuasion (e.g., polite suggestions to share) and sometimes verbal command (e.g., "It's my turn now!"). Based on findings such as these, Maccoby (1988) argued that segregated social groups emerge primarily because children are generally unresponsive to the play and social-influence styles of the opposite sex. Boys, for instance, sometimes try to initiate rough-andtumble play with girls but most (not all) girls withdraw from these initiations, whereas most other boys readily join the fray (Pellegrini & Smith, 1998). Similarly, girls often attempt to influence the behavior of boys through verbal requests and suggestions but boys, unlike other girls, are generally unresponsive to these requests (Charlesworth & LaFrenier, 1983). Segregation may also be related to the formation of the social categories of "boy" and "girl" and a tendency to prefer individuals in the same category. However, same-sex segregation occurs before many children consistently label themselves and other children as a boy or a girl, indicating that social categorization is not likely to be a sufficient explanation for this phenomenon (Maccoby, 1988). The net result of segregation by sex is that boys and gifts spend much of their childhood in distinct peer cultures (Harris, 1995; Maccoby, 1988), and differences in the social styles of boys and girls congeal in the context of these cultures. b. Boys' Peer Relationships. Many features of boys' play, social relationships, and social motives support the position that one-on-one and coalitional male-male

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competition were (and still are) prominent social dynamics during human evolution (Geary, 1998; Geary & Flinn, 2002). As with other species exhibiting male-male competition, boys' activities are more consistently directed toward the achievement of hierarchical dominance than are girls' activities, a sex difference found across culture, age, and social context (e.g., Feingold, 1994; Maccoby, 1988; Whiting & Edwards, 1988). Rough-and-tumble play is one of the earliest social manifestations of physical one-on-one dominance, or at least the practice of this. In situations where activities are not monitored by adults and not otherwise restricted (e.g., a play area that is too small), groups of boys engage in various forms of roughand-tumble play--including playful physical assaults and wrestling--three to six times more frequently than do groups of same-age girls (DiPietro, 1981; Maccoby, 1988). In the United States the sex difference in playful physical assaults and other forms of rough-and-tumble play begin to emerge by about 3 years of age (Maccoby, 1988). The same general pattern is found in other industrial societies and in traditional societies in which it has been studied, although the magnitude of the sex difference varies from one culture to the next (Eibl-Eibesfeldt, 1989; Whiting & Edwards, 1973, 1988). Frequency of boys' rough-and-tumble play peaks between the ages of 8-10 years (Pellegrini & Smith, 1998). At this time, boys spend about 10% of their free time engaged in this form of play. As boys move from childhood to adolescence, the line between play and actual physical aggression begins to blur, and a relation between these activities (e.g., bullying) and social dominance emerges. In one study of 10- to 12-year-old boys, physical aggression and bullying first increased during the early part of the school year, but then decreased as the year progressed--presumably as boys developed stable dominance hierarchies (Pellegrini & Bartini, 2001; see also Savin-Williams, 1987). As physical aggression decreased, affiliative behaviors increased. Unlike younger boys where physical aggression is often associated with unpopularity and social rejection (Newcomb, Bukowski, & Pattee, 1993), these activities in adolescent boys are associated with social dominance, as defined by peers and teachers, and with a higher frequency of dating and higher rated attractiveness by girls (Pellegrini & Bartini, 2001). Affiliativebehaviors following aggressive within-group dominance encounters are common in primate species and function to maintain group cohesion (de Waal, 2000). For humans, the pattern is consistent with the prediction that, for boys, play functions, in part, to practice the formation of competition-related coalitions, that is, to maintain a large enough social group to effectively compete against other groups of boys (Geary, 1998; Geary & Flinn, 2002). The most common venue for the practice of these competencies, such as coordinating group activities, is competitive group-level games. As an example, Lever (1978) found that 10- and 1 I-year-old boys participated in group-level competitive activities, such as football, three times as frequently as did girls. In addition, boys' spontaneous social play involved larger groups, on average, than did girls' social play and involved greater

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role differentiation within these groups. Similar differences have been reported by others (Sandberg & Meyer-Bahlburg, 1994; Sutton-Smith, Rosenberg, & Morgan, 1963). By late adolescence boys' competencies regarding the cooperation and social support needed to function effectively as a competitive coalition, as in the context of team sports, is very sophisticated (Savin-Williams, 1987). The sex difference in one-on-one and group-level competitive play is related, at least in part, to prenatal exposure to androgens (Collaer & Hines, 1995). For example, Berenbaum and Snyder (1995) reported that girls who were prenatally exposed to excess levels of androgens (i.e., congenital adrenal hyperplasia, CAH) engaged in more athletic competition than did their unaffected peers--about three out of four girls affected by CAH engaged in athletic competition more frequently than did the average unaffected girl. This difference, however, was not as large as the difference between unaffected boys and unaffected girls--more than nine out of ten unaffected boys reported engaging in athletic competition more frequently than the average unaffected girl. Hines and Kaufman (1994) found that girls affected by CAH engaged in more playful physical assaults, physical assaults on objects, wrestling, and rough-and-tumble play in general than did unaffected girls, but none of these differences were statistically significant. The lack of significance was possibly due to the testing arrangements used in this study. Here, most of the girls affected by CAH were observed as they played with one unaffected girl, a situation (two girls) that does not typically facilitate rough-and-tumble play. c. Girls' Peer Relationships. In comparison to boys, the social relationships that develop among dyads of girls are more consistently communal, manifesting greater empathy, more concern for the well-being of the other girl, and a greater emphasis on intimacy and social/emotional support (e.g., Maccoby, 1988; Whiting & Edwards, 1988). In addition, girls' social groups tend to be smaller, often including dyads or triads, and are characterized by a motivational disposition centered on cooperation and equality among group members, as contrasted with boys' focus on social dominance (Ahlgren & Johnson, 1979; Knight & Chao, 1989; Rose & Asher, 1999). During the preschool years, the focus of girls' social activities is often sociodramatic play with a family-oriented theme (Pitcher & Schultz, 1983). As they grow older, the focus is more explicitly on the development and maintenance of a small network of friends, with these relationships focusing on interpersonal dynamics (e.g., relationships with other girls or boyfriends) and providing social and emotional support, typically as related to interpersonal conflict (Belle, 1987; Savin-Williams, 1987; Taylor et al., 2000). As described earlier, girls are also competitive, but unlike male-male competition, female-female competition is less physical and functions largely to manipulate and disrupt social relationships through shunning, gossiping, spreading lies, and so on (Crick et al., 1997). As with rough-and-tumble play, relational

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aggression and the associated sex difference emerge by age 3 and continue through childhood, adolescence, and adulthood (Crick et al., 1999). Developmentally, relational aggression becomes increasingly sophisticated. A 3-year-old might try to control the behavior of another child by stating "If you don't play with me, I won't be your friend," whereas an older child will manipulate the social network, for example by spreading rumors (e.g., "She said that you... ") of the girl she is attempting to control. The social sophistication of this form of aggression increases into adolescence and becomes increasingly focused on the disruption of the romantic relationships of other girls (Crick & Rose, 2000). Studies conducted outside of the United States typically find the same developmental pattern and sex difference, although the relevant cross-cultural research is meager in comparison to research on rough-and-tumble play (Crick et al., 1999). The costs associated with being relationally aggressive include higher rates of social rejection and depression (although some relationally aggressive girls are well liked by some other girls; Crick et al., 1999). Relational aggression has benefits in disrupting the social networks of and creating distress in other girls and women--the presumed evolutionary function of this form of aggression. Geary (1998, 2002) argued that the function of women's social networks is to provide social support and stability as well as increased access to important resources, including men. These in turn are associated with better health for these women and, in some contexts, lower mortality and morbidity risks for their children (Flinn, 1999; Geary, 2000; Taylor et al., 2000). In this view, girls and women should highly value the reciprocal and intimate relationships that define and maintain their social networks and should react more strongly than boys and men to the disruption of these relationships; competition for dominance within boys' and men's in-groups result in males being more tolerant of conflicted relationships (de Waal, 1993; Geary & Flinn, 2002). Several analyses of the life events that trigger depressive symptoms in adolescent boys and girls support this position (Bond, Carlin, Thomas, Rubin, & Patton, 2001; Leadbeater, Blatt, & Quinlan, 1995). Both boys and girls experience symptoms of depression following personal failure, such as poor grades. However, a sex difference is found in reactivity to negative interpersonal events: Adolescent girls and women are much more likely to experience symptoms of depression following interpersonal conflict or loss of a significant relationship than are same-age boys and men. In addition, adolescent girls and women often experience symptoms of depression when negative life events affect individuals in their social networks, whereas boys and men typically do not. In fact, adolescent girls apparently are up to four times more likely than same-age boys to experience anxiety and depression as a result of disrupted interpersonal relationships, disruptions that are often the result of relational aggression (Bond et al., 2001). In sum, relational aggression is an effective method for disrupting the romantic and same-sex relationships of other girls and women and can result in significant

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levels of distress, anxiety, and depression for the victims. The costs of relational aggression included the potential for social rejection, but the presumed benefits are increased control of and access to desired social relationships, including romantic relationships.

2. Play Parenting and Family-Oriented Play For most species of primate, play parenting (e.g., caring for siblings) is frequently observed in young females that have not yet had their first offspring, and it is often associated with higher survival rates of the firstborn, and sometimes later born, offspring (Nicolson, 1987). Across five primate species, firstborn survival rates were from two to more than four times higher for mothers with early experience with infant care--obtained through play parenting--than for mothers with no such experience (Pryce, 1993), suggesting that play parenting is indeed a form of practice that refines parenting competencies. Maternal care is also influenced by prenatal exposure to sex hormones and the hormonal changes that occur during pregnancy and the birthing process, such that a combination of early play parenting and hormonal influences contribute to the adequacy of female caregiving in many primate species (Lee & Bowman, 1995; Pryce, 1995). Humans are no exception. In addition to investing more in parenting than men (Geary, 2000), women engage in more play parenting and family-oriented play as children. The sex difference in play parenting is related, in part, to the fact that girls throughout the world are assigned child-care roles, especially for infants, much more frequently than are boys (Whiting & Edwards, 1988). Girls also seek out and engage in child-care, play parenting, and other domestic activities (e.g., playing house)--with younger children or child substitutes, such as dollsm much more frequently than do same-age boys (Pitcher & Schultz, 1983). During the early preschool years, these themes are commonly enacted during solitary play (e.g., playing house with a baby doll), but peers are incorporated into these themes as children become more socially experienced and competent. Beginning around 3-4 years of age, the social-symbolic play of boys tends to focus on issues of physical fighting and competition (e.g., "cowboys and Indians"), whereas that of girls is more commonly focused on family relationships (e.g., "mother and child"; Pitcher & Schultz, 1983). As in other domains, the magnitude of this sex difference varies across age and context. Prior to about age 6, both girls and boys are generally responsive to infants, but after this age, and continuing into adulthood, girls are more responsive to infants and younger children than are boys (Berman, 1986; Edwards & Whiting, 1993). The emergence of this sex difference is related to a significant drop in the frequency with which older boys attend to and interact with infants and younger children (Berman, 1986; Sandberg & Meyer-Bahlburg, 1994). This sex difference has persisted over nearly 40 years of significant change in the social roles and opportunities of women in the United States (Sutton-Smith et al., 1963), and it has

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been found across other industrial societies and in traditional societies in which it has been studied (Eibl-Eibesfeldt, 1989; Whiting & Edwards, 1988). The sex differences in interest in infants, children, and families, as well as in engagement in play parenting, are influenced by the prenatal hormonal environment and by hormonal changes occurring during puberty. Girls affected by CAH show less interest in infants and families and engage in play parenting, among other things, less frequently than their unaffected sisters (Berenbaum & Hines, 1992; Berenbaum & Snyder, 1995; Collaer & Hines, 1995; Leveroni & Berenbaum, 1998). Berenbaum and Hines (1992) compared 5- to 8-year-old girls affected with CAH with unaffected same-sex relatives and found that unaffected gifts played with dolls and kitchen supplies 2 1/2 times longer than did girls affected by CAH. These girls, in turn, played with boys' toys (e.g., toy cars) nearly 2 1/2 times longer than did unaffected girls. The same pattern was found in a follow-up study 3-4 years later (Berenbaum & Snyder, 1995). Furthermore, when allowed to choose a toy to take home after the assessment was complete, unaffected girls most frequently chose a set of markers or a doll to take home, whereas girls affected by CAH most frequently chose a transportation toy (e.g., toy car) or a ball. Unaffected girls also show an increased interest in children following menarche (Goldberg, Blumberg, & Kriger, 1982), whereas the interests of girls affected by CAH remain more malelike (Berenbaum, 1999). 3. Ecologically Related Play Patterns

If men are inherently motivated to attempt to achieve ecological dominance, then sex differences should be found in activities that support this goal. Included among these activities are tool use, hunting, and exploration and control of the wider ecology (e.g., as in control of natural resources). Across traditional societies, men are indeed more likely to use objects as tools (e.g., metal work, weapon making), to hunt, and to travel in unfamiliar territory as related to hunting and warfare (Murdock, 1949). As noted earlier, these activities enable coalitions of men to define and maintain--typically in conflict with other coalitions--a territory for their group and to extract physical (e.g., control of water supply) and biological (e.g., food, medicine) resources from this territory. Although women's gathering contributes to the latter as well, it typically occurs within the confines of the territory maintained by men. In any case, sex differences in play and other developmental activities that would provide the practice needed to refine these competencies are predicted. The associated research base is not as extensive as the base on sex differences in social-developmental activities, but extant research is consistent with the prediction. a. Tool Use. Skilled tool use in adulthood appears to be facilitated by objectoriented play during juvenility. Such play is uncommon in wild primates, except for a few tool-using species, including humans and chimpanzees (Byme, 1995;

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Goodall, 1986). Object-oriented play is often solitary and involves the nonfunctional manipulation of objects, such as throwing them, banging them, and so forth. The function of this play appears to be to learn about the different ways in which various objects can be used, which in turn appears to facilitate later tool use and later problem-solving skills as related to tool use. Chimpanzees that lack objectoriented play during juvenility are, as adults, less successful in problem-solving with objects (Byrne, 1995). As with the chimpanzee, object-oriented play apparently helps children to learn about the physical properties of objects and the different ways in which these objects can be used and classified. For example, preschool children whose play was object oriented had higher scores on tests of spatial cognition (e.g., the ability to mentally represent and mentally manipulate geometric designs) and were better able to sort objects based on, for example, color and shape (Jennings, 1975). Boys and men are consistently found to be more object oriented than girls and women (Willingham & Cole, 1997). In addition, preschool boys learn to use tools more quickly and readily than preschool girls. Chen and Siegler (2000) found that 18-month-old boys were better than same-age gifts at inferring how objects could be used as tools; were more skilled at using objects as tools (e.g., to retrieve a desired toy); and, learned to use objects as tools more quickly, that is, with less practice and less need for adult demonstration. As previously mentioned, gifts affected with CAH show more male-typical object-oriented play than other girls, suggesting that exposure to prenatal androgens may influence this sex difference (Berenbaum & Hines, 1992).

b. Ecological Exploration and Manipulation. Beginning in middle childhood and increasing through adolescence, boys have larger play ranges than girls and explore and manipulate these ecologies (e.g., building things, such as forts) much more frequently than do girls (Matthews, 1992). These sex differences appear to contribute to the sex differences in certain spatial competencies, especially the ability to form a mental representation of the wider ecology, as contrasted with the ability to remember the location of specific objects in this ecology (see Silverman & Eals, 1992). The sex difference in the area of the play range appears to be related, in part, to greater parental restrictions on the ranges of girls than on those of boys. However, a sex difference in the size of the play range is found in the absence of any such restrictions, both in industrial societies and in those traditional societies in which it has been studied (Matthews, 1992; Munroe & Munroe, 1971). In studies of the exploratory play of children in suburban England, for instance, Matthews (1992) found that younger children--both boys and girlsmtended to play within close proximity of one or both of their parents (see also Whiting & Edwards, 1988). In contrast, 8- to 11-year-old children were more likely to play away from home, and the area of the unrestricted play range of boys covered from one and a half to nearly three times the area of the unrestricted play range of same-age gifts.

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Whiting and Edwards (1988) reported a similar sex difference for older children in three separate groups in Kenya as well as for children in Peru and Guatemala. Nonetheless, the age at which this sex difference emerges appears to vary with the ecology of the group. For the Ache, who live in dense, tropical rain forest, the size of the range of boys and girls does not typically diverge until adolescence (Hill & Hurtado, 1996). As with tool use, boys profit more from exploratory behavior than girls. Matthews' (1987) study of the relation between exposure to a novel environment and the pattern of sex differences in the ability to spatially represent this environment illustrates the point. Here, 8- to 1 I-year-old boys and girls were taken on a 1-h tour of an unfamiliar area in suburban England. In one condition, the children were given a map of the entire area and were then taken on the tour, with the guide pointing out various environmental features. In the secondmhigh memory demand---condition, children were given a map of half of the area and their tour was interrupted for 30 min at the halfway point, although the same environmental features were pointed out. At the end of the tour, the children were asked to draw a map of the entire area. The maps of boys and girls did not differ in the overall amount of information provided, but sex differences did emerge for other map features, especially with the high-memory-demand condition. Boys were better able than girls to mentally reconstruct the topography of an unfamiliar environment, retaining general orientation, clustering, and Euclidean (e.g., relative direction) relations among important environmental features. The evidence is not definitive, but studies of CAH suggest hormonal influences on these sex differences. Girls and women with CAH are consistently found to outperform their female relatives on tests of spatial ability and report engaging in more spatial-related behaviors while growing up (Kimura, 1999; Resnick, Berenbaum, Gottesman, & Bouchard, 1986). c. Play Hunting. With very few exceptions (e.g., Hewlett, 1992), hunting is almost exclusively a male activity in traditional societies (Murdock, 1949), and it is a skill that requires years of experience to master (Kaplan et al., 2000). Play hunting has not been directly studied by developmental psychologists, but several patterns support the predicted sex differences. Boys attend to potentially dangerous and wild animals more often than girls do, and in traditional societies engage in play hunting more often than girls. Boys' symbolic play more often involves wild animals than domestic ones (Eibl-Eibesfeldt, 1989). Similarly, the drawings of !Ko (central Kalahari) boys depicted domestic and wild animals about three times more frequently than did girls' drawings. In many traditional societies, a sex difference in the focus of boys and girls daily food-gathering activities emerges in late childhood (Kaplan et al., 2000). For example, until about 10 years of age, both Hadza (Tanzania) boys and girls forage. After this age, boys generally restrict their activities to hunting, despite the fact that their hunting returnsmin

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terms of calories--are much lower than would be the case if they continued to forage (Blurton Jones, Hawkes, & O'Connell, 1997). The long-term benefits can be significant, however, as skilled hunters often have higher status (related to male-male competition) and more reproductive opportunities than other men, as mentioned earlier.

4. Phenotypic Plasticity If one function of the developmental period is to enable inherent neural, cognitive, behavioral, and affective systems to be adapted through developmental activities to local conditions, then plasticity in life history traits should covary with the length of the species' developmental period. If so, then humans should show a higher degree of phenotypic plasticity than any other species, but presumably within the constraints of norms of reaction. As described earlier, many human life history traits do in fact covary with local conditions but these analyses have been limited to physical traits, such as age of maturation (Stearns & Koella, 1986). If the sex differences in social behavior and play patterns are the result of human life history evolution and the influence of sexual selection and other evolutionary pressures, then all the behaviors previously described should evince some degree of phenotypic plasticity. Unfortunately, relevant research is meager, but available studies support the prediction (Low, 1989; MacDonald, 1992). To illustrate, the form and intensity of boys' rough-and-tumble play varies across cultures. In societies characterized by relatively high levels of physical male-male competition, the play fighting of boys tends to be rougher than the play fighting found in other societies. For instance, intergroup aggression is a pervasive feature of Yanomam6 society (Venezuela and Brazil; Chagnon, 1988) and young Yanomam6 boys often play fight with clubs or bows and arrows, practices that are typically discouraged in settings where physical male-male competition occurs infrequently. For the Sioux and Native American tribes that frequently engaged in intergroup hostilities, the activities of young boys were designed to encourage both oneon-one and coalition-based aggression and physical endurance (Hassrick, 1964; Loy & Hesketh, 1995). These activities were often sufficiently violent to draw blood, yet afterward the boys were friendly to each other. This pattern--intense but nonlethal in-group competition and mechanisms (e.g., positive affect during "horse play") to maintain the cohesion of the in-group in the service of intergroup competition----is a predicted feature of male-male competition in humans (Geary & Flinn, 2002). These and other games enable the social, behavioral, and affective systems that support the universal tendency of boys to engage in rough-and-tumble play and coalitional competition to be adapted to local conditions (see Geary, 1998). The mechanisms that support such phenotypic adaptations are not well understood but probably involve a combination of parental and peer influences as well as genotype by environment interactions (Low, 1989; MacDonald, 1992).

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5. Conclusion

The pattern of human developmental sex differences supports all three predictions described previously. First, many sex differences in peer relationships and play mirror adult forms of intrasexual competition, although the findings are more definitive for male-male than for female-female competition given the larger research base for the former (Maccoby, 1988; Crick et al., 1999). More specifically, the basic patterns are consistent with an evolutionary history of one-on-one and coalitional male-male competitions, which very likely involved the use of blunt force and projectile weapons (for further discussion, see Geary, 1998), as related to the achievement of social dominance and resource control. Female-female competition is much less physical but is also related to resource control, although the resources are largely social and relational. Second, consistent with the sex difference in parental effort, girls engage in play parenting and other forms of familyoriented play much more frequently than do boys (Pitcher & Schultz, 1983). Third, boys engage in ecologically related play and other related developmental activities, such as exploration, much more frequently than do girls (Matthews, 1992). Many of these differences are influenced by prenatal exposure to sex hormones and to hormonal changes associated with puberty. Proximate hormonal influences on the expression of sex differences are, in fact, the norm across species, and they are commonly associated with the expression of traits that have been influenced by sexual selection (Geary, 1998). Although not definitive, the results are also consistent with the view that one function of developmental activities is to practice and refine the competencies that covaried with survival and especially reproductive outcomes during human evolution. Reproductive outcomes are emphasized because of low adult mortality rates in traditional societies, such that much of the variance in evolutionary fitness is related to individual differences in mating success and parental success (i.e., keeping children alive) rather than survival per se (Blurton Jones et al., 1997; Hill & Hurtado, 1996; Irons, 1979). Nearly all of the developmental activities described earlier are consistent with survival and reproductive activities in traditional societies and presumably during human evolution. Equally important, the associated competencies (e.g., coalitional competition) become increasingly sophisticated and adultlike during the developmental period and presumably as a result of developmental activities (e.g., Crick et al. 1999; Pellegrini & Bartini, 2001). The majority of these developmental activities are highly social or are related to social issues, as in the relation between hunting competencies, social status, and men's reproductive options (Kaplan et al., 2000). The social nature of developmental activities combined with the more general covariations among length of the developmental period, brain size, and social complexity in primate species (e.g., Joffe, 1997), support the position that various forms of social competition (see Table III) have been driving forces in human evolution and have significantly influenced human life history (Alexander, 1989; Geary & Flinn, 2001).

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The research reviewed here also supports the general prediction that the function of many developmental activities is the accumulation of reproductive potential and the specific hypothesis that sexual selection has been (and continues to be) an important influence on the evolution of sex differences in human life history traits and in developmental activity. In other words, the sex difference in the distribution of reproductive effort across mating and parenting is by definition sexual selection, and the associated sex differences in developmental activity enable the accumulation of competencies that will support sex differences in reproductive effort during adulthood.

VI. Conclusion Life history and sexual selection represent core theoretical principles in evolutionary biology and guide empirical research related to proximate influences and predicted evolutionary functions of developmental traits and patterns of social dynamics, respectively (Andersson, 1994; Darwin, 1871; Roff, 1992). The first goals of the current chapter were to introduce these principles to human developmental scientists and to illustrate their utility for predicting and explaining developmental patterns and sex differences in nonhuman species. The next and more important goal was to construct a theoretical framework for conceptualizing the potential relations between sexual selection in humans and human life history traits, including children's developmental activities. The construction of this framework was guided by the assumptions that social competition, including sexual selection, has been a driving force in human brain, cognitive, behavioral, and social evolution (Alexander, 1989; Geary & Flinn, 2001) and that developmental activities reflect an evolved motivational disposition to practice and thus refine the associated competencies. More precisely, the function of many developmental activities is presumed to be the accumulation of reproductive potential that is then expended in adulthood in the form of reproductive effort: mating, parenting or some combination (Alexander, 1987). As an example, consider that humans share with many other species the same basic life history traits that have been found to covary with male-male competition and an accompanying sex difference in parental effort. Included among these traits are sex differences in life span, maturational dynamics, adult size, and premature death due to male-on-male violence, among other traits (Allman et al., 1998; Leigh, 1996; Wilkins, 1996; Wilson & Daly, 1985). The pattern is clearly consistent with the position that male-male competition--likely exaggerated by female choice--has influenced the evolution of many features of human life history, including components of physical development, reproduction, and a few behavioral triats (i.e., male-on-male aggression; Roff, 1992). At the same time, standard life history analyses do not fully capture the potential influence of social-behavioral

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components of male-male competition and other features of sexual selection on life history evolution, although a few initial analyses have been conducted (e.g., Sawaguchi, 1997). Nor do standard life history analyses capture the importance of developmental activities as related to sexual selection and life history, with very few exceptions (e.g., Collis & Borgia, 1992). When social-behavioral and developmental activities are considered in relation to the importance of coalitional male-male competition and human ecological dominance in traditional societies and presumably during human evolution (Alexander, 1987, 1989; Geary & Flinn, 2001; Wrangham, 1999), many aspects of human life history and associated sex differences fall into place. Sex differences in rough-and-tumble play and participation in sports and other coalitional activities are well documented empirically (e.g., DiPietro, 1981; Lever, 1978; Maccoby, 1988), with the occasional consideration of these patterns as potentially related to male-male competition (Pellegrini & Smith, 1998). However, the consistency of these developmental sex differences with coalitional malemale competition and with one-on-one competition as related to the formation of within-coalition dominance hierarchies has not been fully appreciated. The position here is that these developmental activities are evolved components of human life history that reflect a motivational disposition for boys to engage in activities that prepare them for the forms of one-on-one and coalitional competition that defined male-male competition during human evolution (Geary, 1998). The social nature of male-male competition and associated developmental activities also support Alexander's (1987, 1989) hypothesis that social competition was a driving force during human evolution. Other features of boys' developmental activities, such as object play and play hunting, are in keeping with Kaplan and colleagues' hunting hypothesis-specifically, that hunting/foraging demands were important influences on the evolution of human life history and associated sex differences (Kaplan et al., 2000). The position here is that these activities are indeed important, and related to the evolution of human ecological dominance. At the same time, these activities (e.g., hunting) are aspects of a much broader suite of social, behavioral, and cognitive competencies related to social competition, within (e.g., mating choice) and between groups (Geary & Flinn, 2001). Stated differently, many of these developmental activities enable the accumulation of social, behavioral, and cognitive competencies needed for later reproductive efforts, at least for reproductive efforts in traditional societies and presumably during human evolution. The relation between life history and other components of sexual selection, specifically male choice and female-female competition, are not well understood. For humans, male choice and female-female competition are, nonetheless, integral aspects of sexual selection and, as with male-male competition and female choice, have likely influenced the evolution of human life history traits and developmental activity. As an example, pioneering research by Crick and colleagues on relational aggression provides solid empirical evidence that girls and women

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do indeed compete with one another (Crick et al., 1997, 1999; Crick & Rose, 2000), findings readily interpreted in terms of sexual selection and female-female competition (Geary, 1998, 2002). As with boys' rough-and-tumble play and coalitional games, girls' relational aggression emerges during the preschool years and becomes increasingly sophisticated, socially and cognitively (e.g., in terms of theory of mind), with maturation and practice (Crick et al., 1999). The function of these social activities is to influence and attempt to control social behavior of other girls and women (sometimes boys and men) and with adolescence becomes increasingly focused on romantic relationships, that is, disrupting the romantic relationships of other girls and women. The latter is consistent with the prediction that female-female competition should, in part, be related to paternal investment, that is, developing a spousal relationship and thus access to the man's social and material resources (Geary, 2000; Trivers, 1972). As with boys, many other features of girls' self-initiated activities are interpretable from an evolutionary perspective, although not sexual selection per se. The most obvious of these are play parenting and family-oriented play, which, of course, follow from the sex difference in parental effort. In any case, developmental activities that focus on peer relationships and play parenting both enable the accumulation of competencies related to reproduction in adulthood, female-female competition and parenting, respectively. In sum, basic components of human development, such as length of the developmental period and life span, as well as the details of developmental activities and accompanying sex differences, are readily interpretable from the combined perspectives of life history and sexual selection. Proximate factors, such as gender categorization (Eagly, 1987), are potentially important influences on the expression of many sex differences. Indeed, phenotypic plasticity in life history traits and forms of sexual selection are predicted and have been illustrated in a few cases (Low, 1989; Steams & Koella, 1986). In other words, many of the proximate influences on sex differences might be studied in terms of the broader framework of phenotypic plasticity as related to life history and sexual selection. In any case, a fully informed, human developmental science must incorporate life history traits and the various components of sexual selection.

ACKNOWLEDGMENTS I thank David Bjorklund, Jennifer Byrd-Craven, Mark Flinn, Mary Hoard, and Amanda Rose for insightful comments on an earlier draft and Robert Sites for locating the illustrations used in Figure 2.

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DEVELOPMENTS IN EARLY RECALL MEMORY: NORMATIVE TRENDS AND INDIVIDUAL DIFFERENCES

Patricia J. Bauer, Melissa M. Burch, and Erica E. Kleinknecht INSTITUTE OF CHILD DEVELOPMENT UNIVERSITY OF MINNESOTA MINNEAPOLIS, MINNESOTA 55455

I. INITIATING THE STUDY OF E A R L Y R E C A L L M E M O R Y A. ELICITED IMITATION AS A N O N V E R B A L M E A S U R E OF R E C A L L B. THE NEURAL SUBSTRATE OF EXPLICIT M E M O R Y II. C H A R A C T E R I Z I N G R E C A L L M E M O R Y IN THE FIRST TWO YEARS OF LIFE A. D E V E L O P M E N T A L CHANGES IN THE PREVALENCE AND ROBUSTNESS OF LONG-TERM R E C A L L B. E X P E R I M E N T A L MANIPULATIONS AFFECTING INFANTS' AND YOUNG CHILDREN'S R E C A L L III. INDIVIDUAL DIFFERENCES IN LONG-TERM RECALL: CHILDREN'S GENDER, CHILDREN'S L A N G U A G E PROFICIENCY, AND VARIABILITY IN INITIAL LEARNING A. EFFECTS OF CHILDREN'S GENDER B. EFFECTS OF CHILDREN'S PRODUCTIVE AND RECEPTIVE LANGUAGE C. EFFECTS OF INDIVIDUAL VARIABILITY IN INITIAL LEARNING D. S U M M A R Y OF THREE SOURCES OF VARIABILITY IN EARLY RECALL MEMORY IV. INDIVIDUAL DIFFERENCES IN LONG-TERM RECALL: CHILDREN'S T E M P E R A M E N T CHARACTERISTICS A. W H A T IS TEMPERAMENT? B. W H Y MIGHT TEMPERAMENT CHARACTERISTICS BE RELATED TO EARLY R E C A L L M E M O R Y ? C. EVIDENCE OF RELATIONS B E T W E E N TEMPERAMENT CHARACTERISTICS AND M E M O R Y IN INFANCY AND THE PRESCHOOL YEARS D. RELATIONS B E T W E E N T E M P E R A M E N T AND M E M O R Y IN THE 2ND YEAR: A G E - R E L A T E D DIFFERENCES E. RELATIONS B E T W E E N TEMPERAMENT AND MEMORY: DIFFERENCES WITHIN SUBJECTS F. S U M M A R Y

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V. CHILDREN'STEMPERAMENTAND MOTHERS' LANGUAGEAS INTERACTING SOURCES OF INDIVIDUALDIFFERENCES IN LONG-TERMRECALL A. RELATIONSBETWEENMATERNALLANGUAGEAND OLDER CHILDREN'S MEMORYNARRATIVES B. MATERNALLANGUAGEVARIABILITYWITH YOUNGERCHILDREN IN THE CONTEXTOF ELICITED IMITATION C. RELATIONSBETWEENMOTHERS' LANGUAGE,CHILDREN'S RECALL PERFORMANCE, AND CHILDREN'S TEMPERAMENT CHARACTERISTICS D. SUMMARY VI. CONCLUSIONSAND IMPLICATIONS REFERENCES

The end of the 20th century was witness to nothing short of a sea change in perspective on the mnemonic abilities of infants and very young children. Early in the cognitive era, children younger than 18-24 months were thought to lack the capacity to represent information not available to the senses (i.e., to mentally r e p r e s e n t it; Piaget, 1952). As a consequence, infants were thought to live in a "here and now" world that included physically present entities but which had no future and no past. With the advent of techniques for probing the minds of infants via the eyes of infants (e.g., visual paired comparison: Fantz, 1956), the assumption of an ahistorical infancy began to be questioned. By the early to middle 1970s, it was apparent that infants habituated (Friedman, 1972), could be conditioned (Rovee & Fagen, 1976), and could recognize some types of stimuli (e.g., faces) even after a delay (Fagan, 1973). These demonstrations made clear that even in the first months of life, infants have significant mnemonic competence. Nevertheless, conspicuously absent from the literature was compelling evidence of the capacity for recall and, in particular, for long-term recall. In contrast to recognition, recall involves "... accessing (bringing to awareness) a cognitive structure pertaining to a past experience not currently available to perception" (Mandler, 1984, p. 79). Recall is the process in which we engage when we retrieve from memory what we had for dinner last evening or where we spent the most recent summer vacation. Perhaps especially against the backdrop of demonstrations of other types of memory, the dearth of evidence of recall by infants and young children led to suggestions that this particular mnemonic ability was late to develop. The suggestion that infants and young children lack an ability that most adults take for granted seemingly was supported by at least two other sources: research on adults' memories of childhood and cognitive neuroscience. Moreover, the

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assumptions were hard to examine for methodological reasons. Beginning in the mid-1980s, however, the suggestion that long-term recall ability is late to develop was challenged and found wanting. In this chapter, we discuss methodological innovations that enabled examination of recall abilities in pre- and early verbal children. We then review the empirical literature illustrating developmental changes in this important cognitive ability during the first 2 years of life, as well as neurodevelopmental changes that may underlie them. We first focus on normative trends of development in the first 2 years and then on individual differences that influence the normative trends. Specifically, we examine possible influences on early recall memory of children's gender and their levels of proficiency with language as well as children's temperament characteristics. Whereas there appear to be few associations between gender and recall and language and recall, we present evidence that temperament characteristics have direct relations with developing recall memory skills. Children's temperament characteristics also work to influence the verbal support that children's mothers provide them in mnemonic contexts, which in turn affects children's recall performance. Finally, we discuss the implications of the findings for our understanding of early developmental change as well as of later developing episodic and autobiographical memory competence.

I. Initiating the Study of Early Recall Memory One source of the assumption that infants and very young children are unable to recall the past was research on adults' memories of childhood. Since Sigmund Freud first labeled it in 1905, the phenomenon of infantile amnesia has been one of the great curiosities in the field of memory. Infantile or childhood amnesia refers to the relative paucity among adults of verbally accessible memories from the first years of life (e.g., Pillemer & White, 1989) and fewer memories from ages 3 89 years than expected based on forgetting alone (e.g., Rubin, 1982). In his interviews with adult patients, Freud noticed that few had memories from their early years. The memories they did have were sketchy and incomplete. Freud termed this phenomenon infantile amnesiac"the amnesia that veils our earliest youth from us and makes us strangers to it" (1916/1966, p. 326). In the years since Freud identified infantile amnesia, many investigators have studied adults' memories of their childhoods. This work has yielded the robust finding that among adults, in Western culture, the average age of earliest verbalizable memory is 3 5 years (Dudycha & Dudycha, 1941; see West & Bauer, 1999, for a review of contemporary research; see, for example, Mullen, 1994, for different estimates of earliest memories among non-Western adults). Over the course of his career, Freud developed two hypotheses regarding the source of infantile amnesia. The most widely known is his hypothesis that early

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memories exist but are repressed (Freud, 1916/1966). His other suggestion was that early memories are mere fragments of experiences that are not organized in a coherent, adultlike fashion (Freud, 1905/1953). Piaget's (1952) characterization of the first 18-24 months of life as a period devoid of a means to represent absent objects (and events) was a natural ally for this suggestion. Also consistent were early conclusions (based largely on research with rats; see C. A. Nelson, 1995, 1997, for discussion) that the neural substrate supporting recall was functionally late to develop. Even as tenets attributed to Piagetian theory were being challenged (e.g., Gelman & Baillargeon, 1983), and as data suggesting earlier functional maturity of the neural substrate of recall were accumulating (e.g., Schacter & Moscovitch, 1984), the suggestion that infants were unable to recall previous experiences prevailed. Indeed, in the early 1980s, there were virtually no data on long-term recall memory in children younger than age 3 years. From the work of Katherine Nelson and her colleagues (e.g., Nelson & Gruendel, 1981, 1986), we knew that children 3 years and older had impressive long-term recall abilities (e.g., Fivush, 1984; Hudson, 1986; Nelson & Gruendel, 1981). By that age, children evidence well-organized representations of familiar events, such as going to fast-food restaurants. Diary reports and naturalistic observations also revealed evidence of recall memory in preschool-age children (e.g., Ratner, 1980). However, recall of the past by children younger than 3 years had not been examined. The major reason for the absence of research on children younger than 3 years was that conceptually and methodologically, the ability to recall was associated with the ability to provide a verbal report. Because children 3 years and older could use language to talk about the past, they clearly could recall it. However, 1- to 2-year-old children are not particularly adept at language. Even by late in the 2nd and into the 3rd year, when children have made substantial gains in language, they have difficulty using their language to talk about past experiences. Not until children are 3 years of age and older do they become reliable partners in conversations about past events (e.g., Fivush, Gray, & Fromhoff, 1987). This reality, and that the average age of adults' first memories is 3 71 , reinforced the notion that it was not until the 4th year of life that children develop the ability to recall. Such was the backdrop against which research on recall of events by infants and very young children began. The first step in this research was to establish a means of testing recall nonverbally. Imitation-based procedures fit the bill. Instead of being required to tell what they remember, in imitation procedures, children are afforded the opportunity to show what they remember. Specifically, elicited and deferred imitation involve using objects to produce a unique action or sequence of actions, and then allowing the infant or child to imitate the actions either immediately, or after a delay, or both. Examples of stimuli used in imitation-based studies in our laboratory are provided in Table I. Imitation after exposure to a model was viewed by Piaget (1952) as one of the hallmarks of the capacity for representation.

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TABLE I Examples of Stimuli Used in Imitation-Based Studies

Example of two-step sequence Sequence: Make Big Bird turn on the light. Materials: Small plastic car containing "Big Bird" character; clear plastic L-shaped base; plastic plunger extended from one end of the horizontal section of the base; small light bulb on end of horizontal section opposite plunger. Steps: (1) Putting the car in the base (i.e., inserting it down the vertical section of the base), and (2) pushing in the plunger, thereby causing the car to roll to the other end and illuminate the light bulb. Example of three-step sequence Sequence: Make a rattle. Materials: Two rectangular nesting cups; wooden block. Steps: (1) Putting the block into one nesting cup, (2) inverting one nesting cup into the other, and (3) shaking the cups to make them rattle. Example offour-step sequence Sequence: Make a gong. Materials: Base resembling the support for a swing set; small cup attached to one post of the swing-set-shaped base; bar resting in the cup; metal plate with a lip; small plastic mallet. Steps: (1) Lifting the bar from the cup, (2) putting the bar across the posts (to form a crosspiece), (3) hanging the plate from the bar, and (4) hitting the plate with the mallet, thereby causing it to ring.

A. ELICITED IMITATION AS A NONVERBAL MEASURE OF RECALL T h e i m i t a t i o n - b a s e d p r o c e d u r e s u s e d in our laboratory involve several steps. First, before s e q u e n c e s of actions are m o d e l e d , the associated objects or props are given to the infant or child for a baseline period, during w h i c h s p o n t a n e o u s p r o d u c t i o n of the target actions and s e q u e n c e s is assessed. After the baseline, e x p e r i m e n t e r s label the s e q u e n c e to be p r o d u c e d and then narrate their actions as they use the props to enact the sequence. In situations in w h i c h i m m e d i a t e recall is assessed, after m o d e l i n g , the e x p e r i m e n t e r returns the props to the infant or child and e n c o u r a g e s imitation. 1 M e m o r y is inferred if imitative levels of p r o d u c t i o n of the target actions and s e q u e n c e s e x c e e d baseline levels. To test retention over time, the infant or child returns to the l a b o r a t o r y after a delay at w h i c h point the props are p r o v i d e d but the s e q u e n c e s are not r e m o d e l e d . In s o m e studies, verbal r e m i n d e r s 1 In some applications of the imitation task, the opportunity to imitate is deferred for a period of hours (e.g., Meltzoff, 1988b) to months (e.g., Meltzoff, 1995). That is, children are not permitted to imitate prior to imposition of a delay. Although elicited-imitation protocols that permit imitation prior to the delay and those that do not (i.e., deferred-imitation protocols) might by thought to test different underlying abilities (Piaget, 1952), apparently they do not (see Bauer, Wenner, Dropik, & Wewerka, 2000, for extended discussion). Because the term "elicited imitation" characterized protocols both that require deferred imitation and that do not, unless the distinction between protocol types is relevant, for the balance of this chapter we will use the more generic term to refer to the task.

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that the child could, for example, "Use this stuff to make a rattle," are given along with the props; in other studies, only the props serve to remind the children of the events. Higher levels of performance after the delay, relative to baseline, are taken to indicate long-term memory. From each phase of testing we determine both the number of different individual steps or actions of the event produced and, as a measure of organization of the event representation, the number of pairs of steps or actions produced in the target order. There are excellent reasons to believe that elicited imitation serves as a nonverbal analog to verbal report. One very telling observation in this regard is that once children have the linguistic capacity to do so, they talk about the events that they experienced in the context of imitation (Bauer, Kroupina, Schwade, Dropik, & Wewerka, 1998; Bauer, Wenner, & Kroupina, 2002; Bauer & Wewerka, 1995, 1997). That children subsequently are able to talk about events experienced in the elicited-imitation paradigm is strong evidence that the representational format in which the memories are encoded is compatible with language if not itself symbolic (see Mandler, 1998, for further discussion of representational formats). Ideally, the logical argument that elicited imitation is a nonverbal analog to a verbal report would be complemented by evidence showing that conditions that undermine verbal recall ability also produce deficits in imitative performance. To examine this possibility, McDonough, Mandler, McKee, and Squire (1995) tested deferred imitation by adults with amnesia associated with damage to the medial temporal lobe structures implicated in recall memory in particular and declarative, or explicit, memory more generally. 2 Relative to normal adults who readily learned and remembered the multistep sequences, patients with amnesia did poorly. Indeed, they performed no better than control participants who had never seen the sequences demonstrated. This finding strongly suggests that imitation procedures tap explicit memory, which gives rise to the capacity to recall (for further development of the argument that elicited imitation taps explicit mnemonic processes, see, for example, Bauer, 1996, 1997, 2002b; Carver & Bauer, 2001; Mandler, 1990; Meltzoff, 1990). The findings of McDonough et al. ( 1 9 9 5 ) make clear that damage to the medial temporal lobe structures implicated in amnesia impairs adults' recall memory as measured by the elicited-imitation task. A logical expectation based on this 2 Declarative, or explicit, memory involvesthe capacity for conscious recognition or recall of names, places, dates, events, and so on (e.g., Squire, 1982, 1986). It is the type of memory that most of us refer to when we talk about "memory for" or "remembering" a past event. Declarative, or explicit, memory is characterized as fast (e.g., supporting one-trial learning), fallible (e.g., memory traces degrade, retrieval failures occur), and flexible (i.e., not tied to a specific modality or context). In contrast, nondeclarative, or implicit, memory represents a variety of nonconscious abilities, including the capacity for learning habits and skills, some forms of classical conditioning, and priming (i.e., facilitated processing of a stimulus as a result of previous exposure to it). It is characterized as slow (i.e., with the exception of priming, it results from gradual or incremental learning), reliable, and inflexible (Squire, Knowlton, & Musen, 1993). For ease of exposition, throughout this chapter, we use the term explicit to refer to memory characterized as either declarative or explicit.

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finding is that the developmental status of the neural circuitry also would impact performance. In other words, neurological development should place a lower limit on the capacity for recall memory as evidenced by performance on the elicitedimitation task. In the next section we provide a brief overview of the neural substrate implicated in explicit memory in general and recall memory in particular and of our emerging, yet still incomplete, understanding of the course of development of the circuitry. B. THE NEURALSUBSTRATEOF EXPLICITMEMORY In adult humans, recall memory, and in particular long-term recall, is thought to be dependent on particular neural structures and their connections. The literature suggests that the formation, maintenance, and subsequent retrieval of memories over the long term depends on a multicomponent network involving temporal (including the hippocampus, entorhinal, and perirhinal cortices) and cortical (including prefrontal cortex and limbic/temporal association areas) structures (e.g., Bachevalier & Mishkin, 1994; Murray & Mishkin, 1998). The medial temporal lobes are involved in initial encoding and consolidation of memory traces, the cortical association areas are the presumed long-term storage sites, and the prefrontal cortex is implicated in retrieval of memories from long-term stores (e.g., Bachevalier & Mishkin, 1994; Eichenbaum & Cohen, 2001; Squire, Knowlton, & Musen, 1993). One of the major functions of the hippocampus is to bind together distributed sites in the neocortex that together represent an event (e.g., Kandel & Squire, 2000; Moscovitch, 1992; Zola-Morgan & Squire, 1990). Specifically, inputs from multiple neocortical association areas are thought to converge on parahippocampal structures (e.g., entorhinal cortex) where, without further processing, they are maintained temporarily and then only as isolated elements. The processing that creates relations among the elements is carried out by the hippocampus (Eichenbaum & Cohen, 2001). The organization and consolidation of a memory trace involves neurochemical and neuroanatomical changes that occur over the course of hours to months and eventually render the memory representation independent of the hippocampus (Squire, 1992). Consistent with this suggestion, patients with damage to medial temporal structures have impairments in storing new information but not in retrieving information stored long ago (e.g., Squire, 1992; Squire & Zola-Morgan, 1991; Zola-Morgan, Squire, & Amaral, 1986). Impaired storage is particularly pronounced for episodic features of new events, such as the specific time or place of the experience (e.g., Squire, 1986; Vargha-Khadem et aL, 1997). Behavioral (e.g., Jetter, Poser, Freeman, & Markowitsch, 1986; Markowitsch, 1995) and neuroimaging (e.g., Cabeza et al., 1997; Kapur et al., 1995) evidence implicates the prefrontal cortex in retrieval of memories from long-term stores. The structure seems to play a particularly prominent role in retrieval of episodic features, such as temporal information (e.g., Janowsky, Shimamura, & Squire,

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1989; Lepage & Richer, 1996; Squire, 1982, 1986) and the specific identity of previously encountered elements (e.g., words lists; Dywan & Jacoby, 1990; Jacoby, 1991). Autobiographical or personal memory may be especially dependent on the prefrontal cortex (e.g., Barnett, Newman, Richardson, Thompson, & Upton, 2000). Moreover, recall of episodic information requires not only the prefrontal, association, and temporal structures themselves, but also intact connections between them (Yasuno et al., 1999). A logical implication of this analysis is that neurological development will limit the capacity for long-term recall. That is, the capacity to recall experiences after a delay will emerge only once the neural structures in the explicit memory network, as well as the connections between them, reach a minimum level of functional maturity. In primates, most of the hippocampus matures early (Serres, 2001). However, several structures are later to develop, including the dentate gyrus of the hippocampus (a critical link in the circuit that connects parahippocampal structures to the CA3 and CA1 regions of the hippocampus), the frontal cortex (implicated in retrieval from long-term stores), and temporal-cortical connections. The full network begins to coalesce in the second half of the 1st year of life (C. A. Nelson, 1995, 1997; Schacter & Moscovitch, 1984) and it continues to develop for months thereafter (Carver & Bauer, 2001; Nelson & Webb, 2002; Serres, 2001). Consistent with these suggestions, in the next section we review evidence that long-term recall memory is newly or recently emergent at 9 months of age and that it increases in prevalence and robustness over the course of the 2nd year of life.

II. Characterizing Recall M e m o r y in the First Two Years of Life A. DEVELOPMENTALCHANGESIN THE PREVALENCEAND ROBUSTNESS OF LONG-TERMRECALL

1. Age-Related Changes in the Prevalence of Long-Term Recall Some of the evidence that long-term recall memory processes are newly (or recently) emergent at 9 months, and that they become increasingly prevalent over the 2nd year of life, comes from a study by Carver and Bauer (1999). We tested 9-month-olds' recall of two-step sequences over a 1-month delay. We used multistep sequences (rather than single actions: e.g., Meltzoff, 1988a,b) because reproduction of multistep sequences in the correct temporal order provides especially compelling evidence of recall, as opposed to recognition (e.g., Bauer, 2002b). Consider that to reproduce an ordered sequence the infant or child cannot rely on recognition: Once modeling or demonstration of the sequence is complete, information about the order in which the sequence unfolded is not perceptually available. To reproduce an ordered sequence then, the infant or child must encode order information during demonstration of the event, and later retrieve

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the information from a representation of the event, in the absence of ongoing perceptual support (see Bauer, 1996, 1997, 2002b, for further discussion). In this requirement, the task is analogous to verbal recall (Mandler, 1990). Because reproduction of multistep sequences in the correct temporal order provides the strongest evidence of recall, in discussing normative changes in long-term recall memory over the first 2 years of life, we focus on the extent to which infants and children adhere to the target order in their reproductions of event sequences. In the Carver and Bauer (1999) study, as a group the 9-month-olds recalled the individual actions of the events. That is, after the 1-month delay, the infants produced a larger number of the individual actions of the events to which they had been exposed, relative to new, control events. However, only 45% of the infants evidenced temporally ordered recall of the sequences. We since have replicated this pattern in two independent samples. In studies by Bauer, Wiebe, Carver, Waters, and Nelson (2002) and Bauer, Wiebe, Waters, and Bangston (2001), 46 and 43% of 9-month-olds evidenced ordered recall, respectively. Thus, at 9 months, individuals differ considerably in long-term ordered recall. Nevertheless, the behavior of 9-month-olds differs from that of 6-month-olds, only a small proportion (25%) of whom evidence ordered recall over a delay as brief as 24 h (Barr, Dowden, & Hayne, 1996). Together these findings suggest that long-term ordered recall is newly (or recently) emergent at around 9 months of age. Over the 2nd year of life, long-term recall ability becomes increasingly prevalent. In a large-scale study of the parameters of remembering and forgetting in the 2nd year of life (Bauer, Wenner, Dropik, & Wewerka, 2000), we enrolled children at the ages of 13, 16, or 20 months and tested them for recall of multistep sequences after delays of 1, 3, 6, 9, or 12 months (delay was between-subjects). Table II shows the percentages of children of each age who, at delayed testing, evidenced ordered recall (i.e., higher performance on previously experienced as compared with new control sequences). An asterisk indicates that the number of children with the pattern is greater than the number that would be expected by chance. In contrast to 9-month-olds, only roughly half of whom evidence ordered recall after 1 month (Bauer et al., 2001; Bauer, Wiebe, et al., 2002; Carver & Bauer, 1999), after 1 month, high, and roughly comparable, percentages of 13-, 16-, and 20-month-olds evidenced ordered recall. As the delay interval increased, fewer children maintained information about the order of the events; the younger the children were at the time of experience of the events, the steeper was the forgetting function. These data thus indicate age-related increases in the prevalence of longterm ordered recall. In summary, 9 months seems to represent a developmental point at which a newly (or recently) emergent behavior is seen in a subset of the population. Only 4 months later, at 13 months, the behavior is readily observed. Over the course of the 2nd year, the temporal extension of the ability increases, such that by 20 months, almost 70% of children show temporally ordered recall after as many as 12 months.

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T A B L E II Percentages of Children S h o w i n g Evidence of Ordered Recall Over Retention Intervals of 1 to 12 Months a Delay interval (in months) A g e at experience

1

3

6

9

12

13 months

78*

67

39

44

39

16 months

94*

94*

72*

50

61

20 months

100"

100"

83*

78*

67*

a Results from Bauer, Wenner, Dropik, and Wewerka (2000). Note: A total of 360 children participated, 180 of whom were 16-month-olds. All of the 13-month-olds and half of the 16-month-olds were tested on sequences three steps in length; all of the 20-month-olds and half of the 16-month-olds were tested on sequences four steps in length. Differences in sequence length accommodated age-related changes in the lengths of sequences that children can accurately imitate (see Bauer, 1995, 1996, 1997, for reviews). At each of three sessions, spaced 1 week apart, the children were exposed to the same six event sequences. Three of the sequences they never were permitted to imitate; three of the sequences they were permitted to imitate one time, at the end of the third exposure session. The children returned for delayed-recall testing after delay intervals of either 1, 3, 6, 9, or 12 months. At the delayed-recall session, the children were tested for recall of the six sequences to which they previously had been exposed as well as on three new sequences, as a within-subjects control. Since performance did not differ as a function of whether imitation was permitted, the data are collapsed across this manipulation. The specific values are for 16-month-olds tested on four-step sequences; the pattern applies to both groups of 16-month-olds (see Bauer et al., 2000, for details). Asterisks indicate that the number of children with higher levels of performance on previously experienced rather than on new, control sequences was reliably greater than chance. Because determination of chance levels is affected both by the number of observations and by the number of tied observations, identical values will not necessarily yield identical outcomes (e.g., 13-month-old 3-month delay and 20-month-old 12-month delay).

2. Age-Related Changes in the Robustness of Long-Term Recall In addition to evidence of age-related increases in the prevalence of long-term recall across the first 2 years of life, our research has revealed changes in its robustness. At 9 months of age, when the capacity is newly or recently emergent, long-term recall is fragile, apparently depending on multiple experiences of events. If infants receive fewer than three exposures to events prior to imposition of the delay, a maximum of only 21% demonstrate ordered recall over 1 month (Bauer et al., 2001). In contrast, by 13 months, only a single experience is necessary for ordered recall (Bauer & Hertsgaard, 1993). By 13 months of age, children no longer require multiple experiences to maintain temporally ordered event representations over a delay. Nevertheless, the robustness of long-term recall memory changes over the 2nd year of life. In the Bauer et al. (2000) study, across the delay intervals of 1, 3, 6, 9, and 12 months, older children recalled more than younger children. Age effects were especially apparent in recall of temporal order and under conditions of greater cognitive demand. Specifically, in regression analyses, age contributed more unique variance in children's ordered

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recall than in recall of the individual actions of the sequences. Age contributed more variance at the longer retention intervals of 6, 9, and 12 months, relative to the shorter retention intervals of 1 and 3 months. In addition, age effects were especially apparent when children's recall was supported by the event-related props alone. Age differences diminished when verbal reminders were provided (i.e., after a period during which recall was prompted by event-related props alone, the experimenter provided a verbal reminder in the form of the label used to introduce the sequence at each exposure session). 3. Summary Together, changes in the prevalence and robustness of long-term recall late in the 1st year and throughout the 2nd year of life suggest consolidation of mnemonic function in this time frame. Whereas at 9 months individual differences in recall over 1 month are the rule, by early in the 2nd year they are the exception. Over the 2nd year of life, the capacity to recall over yet longer delays consolidates such that by 20 months, a large proportion of children demonstrate ordered recall after as many as 12 months have passed.

B. EXPERIMENTALMANIPULATIONSAFFECTINGINFANTS'AND YOUNGCHILDREN'SRECALL Although there are regular, normative changes in long-term recall memory in the first 2 years of life, a child's age is not the sole determinant of whether or for how long she or he will remember. Moreover, within an age group, there are individual differences in how much children remember. In this section we consider some of the experimentally controlled determinants of long-term recall memory. In section III we discuss individual differences in performance. A number of factors have been experimentally manipulated and found to influence children's recall over both the short and the long term. Because the influence of these factors has been reviewed in detail elsewhere (e.g., Bauer, 2002a,b; Bauer et al., 2000), we summarize them briefly here, highlighting: (a) the nature of temporal connections in events, (b) the number and timing of experiences of events, (c) active participation in events, and (d) verbal reminders of to-be-remembered events. First, children's ordered recall of event sequences is facilitated by enabling relations. Enabling relations are said to exist when, to achieve a particular endstate or goal, the actions in the sequence must occur in a particular temporal order. For example, to Make Big Bird turn on the light as described in Table I, the car must be in the base before the plunger is pushed. If the steps are performed in alternate order (i.e., push in the plunger and then put the car in the base), then the light bulb would not illuminate. Children's ordered recall of sequences constrained by enabling relations is better than their ordered recall of sequences lacking enabling

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relations (which are ordered arbitrarily). The effect is apparent both at immediate testing (e.g., Bauer, 1992; Bauer & Thai, 1990) and after short delays (e.g., Barr & Hayne, 1996; Bauer & Hertsgaard, 1993; Mandler & McDonough, 1995). As delay length stretches from weeks to months, although children still remember the events, the facilitating effects of enabling relations diminish (Bauer et al., 2000). Whereas even the youngest children tested accurately recall sequences constrained by enabling relations (e.g., Bauer & Mandler, 1992; Carver & Bauer, 1999; Mandler & McDonough, 1995), not until the second half of the 2nd year of life do children reliably reproduce arbitrarily ordered sequences in the correct temporal order (Bauer, Hertsgaard, Dropik, & Daly, 1998; Wenner & Bauer, 1999). (See Bauer, 1992, 1995, and Bauer & Travis, 1993, for discussions of the means by which enabling relations in events may influence ordered recall.) Second, the number and timing of experiences of events also influence children's recall. Repeated experience aids memory both in terms of the amount of information remembered, and in terms of the length of time over which events are recalled (e.g., Bauer, Hertsgaard, & Wewerka, 1995; Bauer et al., 2001; Fivush & Hamond, 1989). For children as young as 13 months, repeated experience is not necessary for recall over the short term (Bauer & Hertsgaard, 1993; Bauer et al., 1995). Nevertheless, particularly over the longer term, repeated experience facilitates recall. For example, in the Bauer et al. (1995) study, for events experienced only once, performance after a 1-month delay was substantially lower than performance after a 1-week delay. In contrast, events experienced three times before imposition of a 1-month delay were well recalled. Notably, recall after 1 month of events experienced three times was comparable to that after 1 week of events experienced only once (see Fivush & Hamond, 1989, for similar effects with 24and 29-month-olds). The timing of exposures to events also affects the efficacy of repeated experience. Hudson and Sheffield (1998) found that the effects of reexperiencing events after 8 weeks were more pronounced than reexperiencing the events after 15 min or after 2 weeks. Events need not be reexperienced in their entirety for recall to be facilitated. Simply showing children a subset of events (i.e., three of an original six experienced) facilitates memory for the entire set, presumably through spreading activation (Sheffield & Hudson, 1994). Third, active participation in events is not necessary for later recall (e.g., Barr & Hayne, 1996; Bauer et al., 2000; Meltzoff, 1995). Nevertheless, active participation in the form of imitating to-be-remembered events is associated with better recall, at least over short delays (e.g., Bauer et al., 1995). As is the case for effects of the temporal structure of event sequences, over longer delays, effects of active participation dissipate. For example, in the Meltzoff (1995) study, recall after 2 and 4 months was comparable for 14- and 16-month-old children who were and were not allowed to imitate prior to imposition of the delays. Similarly, in the Bauer et al. (2000) study, regardless of the length of the delay (i.e., 1 to 12 months), recall was comparable for events that 13- to 20-month-old children had and had not been allowed to

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imitate prior to imposition of the delays. Thus, active participation in the form of imitation enhances recall over the short term but not necessarily over the long term. Fourth, in children preschool age and older, cues or reminders of previously experienced events facilitate memory retrieval after a delay (e.g., Hudson & Fivush, 1991). Verbal reminding also aids memory retrieval over delay intervals of many months in children as young as 13 months at the time of experience of novel events (Bauer et al., 2000; see also Bauer et al., 1995). Indeed, as noted earlier, verbal reminding has the effect of reducing age-related differences in the amount of information that young children recall after a delay (Bauer et aL, 2000). Critically, the effects of verbal reminding cannot be attributed to "suggestion" of plausible event sequences: Children do not generate the target actions and sequences of events for which they have been provided a verbal label but to which they have not been exposed (Bauer et al., 1995, 2000). That memories can be triggered by verbal reminders is particularly important to recall after long periods of time: Over significant delays, regardless of age, little that is not reminded is retrieved (e.g., Hudson & Fivush, 1991).

III. Individual Differences in Long-Term Recall: Children's Gender, Children's Language Proficiency, and Variability in Initial Learning Knowledge of the factors that affect long-term recall permits prediction of group trends in mnemonic performance. For example, we expect high levels of recall when children are tested under "optimal" conditions such as when to-beremembered events are constrained by enabling relations; when children have well-timed, multiple experiences of events; when children are permitted active participation at encoding; and when verbal reminders are provided at the time of retrieval. Conversely, we expect less accurate recall of sequences lacking enabling relations, of which children receive only a single observational experience, and of which they are given limited reminders. Whereas knowledge of what factors affect long-term recall permits substantial leverage in predicting group trends, it constitutes only one category in the catalog of the determinants of infants' and young children's recall memory. An equally important category of explanation is of individual differences in performance. Early in development, when long-term recall ability is newly emergent, individual differences are apparent in whether or not children show evidence of memory. That is, roughly 50% of 9-month-olds demonstrate ordered recall whereas roughly 50% do not (e.g., Bauer et al., 2001). As recall ability stabilizes and becomes more prevalent, individual differences take the form of variability in the a m o u n t remembered. For example, in the Bauer et al. (2000) study, under "optimal" conditions (i.e., verbally cued memory for enabling sequences that children had experienced three times and imitated one time prior to imposition of a 1-month delay),

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even among children in whom long-term recall ability is prevalent and robust (20-month-olds), some children recalled all four possible target actions whereas others recalled as few as one. Similarly, some children evidenced perfect temporally ordered recall whereas others revealed no evidence of ordered recall. What accounts for such wide variability in performance? Given that we cannot explain it by appeal to experimentally controlled factors (i.e., all of the children in this cell of the design were subjected to the same encoding and test conditions), we must look to characteristics that vary across individuals. Among preschool age and older children, individual differences in memory are associated with factors such as strategy use, metamemory, content knowledge, speed of processing, and overall intelligence (see Schneider & Bjorklund, 1998, for a review). However, research on individual differences in early recall has not concerned the factors familiar in research with older children. In some cases, the reason is obvious. Very young children are not very strategic in their memory performance (although see DeLoache, Cassidy, & Brown, 1985) nor are they skilled at describing their own or others' cognitive processes. For these reasons, examinations of variability in strategy use and metamemory as sources of variability in very young children's recall are unlikely to be productive. Differential knowledge of content may well influence young children's recall in the same manner as it influences older children's. This possible source of individual differences has not been pursued, however, primarily because we do not have adequate tools for evaluating semantic memory content or organization in pre- and early verbal children (although see Bauer, 1993, for gender-differential recall of masculineand feminine-typed event sequences by 25-month-old boys, possibly as a function of greater knowledge of same-sex behaviors). Finally, although factors such as speed of processing and overall intelligence have been addressed in the literature on recognition memory in infants (e.g., Fagan, 1984; Rose, Feldman, & Wallace, 1992), they have not been examined as possible sources of variability in young children's recall memory. In the literature on early recall memory, two possible sources of differences in performance have been studied: one is a potential source of group differences, namely, children's gender, and the other is a potential source of individual differences, namely, children's language competence. We review the literatures concerning each of these sources in turn. We then discuss a third possible source of influence, namely, individual variability in initial learning of events. A. EFFECTSOF CHILDREN'SGENDER In the case of gender, the expectation of possible systematic variability in longterm recall memory performance stems from suggestions of faster maturation of girls relative to boys throughout infancy (e.g., Hutt, 1978; although see Reinisch, Rosenblum, Rubin, & Schulsinger, 1991). Although differential maturity provides a reasonable basis for expecting gender differences, few relations have been found.

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For example, in 16 of 34 experiments published from our laboratory since 1987, tests for possible effects of gender were conducted. In 12 of the 16 cases, levels of performance by girls and boys did not differ. In four experiments, effects of gender were detected. Three of the four cases involved 9-month-olds, an age at which long-term recall ability is recently or newly emergent. In two cases, the effect of gender favored boys (Bauer et al., 2001; Experiments 1 and 2); in one case, the effect favored girls (Carver & Bauer, 1999). In the final experiment (Bauer, Hertsgaard, et al., 1998), 28-month-old children were challenged to reproduce lengthy, arbitrarily ordered event sequences (a late-developing skill; e.g.,Wenner & Bauer, 1999). In this case, the advantage was for girls relative to boys. Others using imitation procedures with children in this age range have had similar actuarial experience. A sample of 31 published experiments from the laboratories of Harlene Hayne and Andrew Meltzoff, both of whom employ imitation paradigms with children aged 2 years and younger, yielded six cases in which tests for possible gender effects were conducted. All yielded null effects. The remaining 25 experiments did not report tests for possible effects of children's gender. One reason that gender has not been examined more systematically is that studies of early recall memory typically include small samples of children. Across the 65 experiments examined for treatment of possible effects of gender (i.e., 34 from our laboratory and 31 from the laboratories of Hayne and Meltzoff), sample sizes ranged from 8 to 32 participants per cell of the research designs; the modal sample size was 12 participants. To divide such small samples in half to examine gender differences is to invite nulluor worse, spuriousmfindings. However, with larger samples, possible gender-related differences can be examined, perhaps profitably. Fortunately, the Bauer et al. (2000) study provides an opportunity to address the issue. The sample included 360 children, roughly half girls and half boys. In the Bauer et al. (2000) study, 185 girls and 175 boys were tested for longterm recall after delays of 1, 3, 6, 9, or 12 months. Examination of gender-related patterns of performance in the large sample thus affords a relatively definitive test of gender as a potential source of variability in young children's recall memory performance. We conducted separate analyses for the 20- and 16-month-olds, tested on four-step event sequences, and for the 13- and 16-month-olds, tested on three-step event sequences. As noted earlier (see section II.A.2), children's long-term recall was tested under two conditions. First, for each sequence, the children were provided with the props for the event. After a child-controlled period of manipulation of the props, the experimenter provided a verbal reminder in the form of the title given the sequence at the time of its introduction (e.g., "You can use this stuff to make a rattle"). When the children were prompted only by the event-related props, some gender differences emerged. For the children tested on four-step sequences, in the 9-month delay condition, girls produced fewer of the individual target actions of the events, relative to the boys (M = 1.15 and 1.73, respectively). Lower levels of production of the individual target actions of the events translated into lower levels

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of ordered recall by the girls, relative to the boys (M = 0.25 and 0.50, respectively). Among the children tested on four-step event sequences there were no reliable gender effects in the 1-, 3-, 6-, or 12-month delay conditions. Moreover, when the children were supported both by the event-related props and by verbal reminders of the events, girls and boys did not differ, even in the 9-month delay condition. For the children tested on three-step sequences, prior to provision of the verbal reminders, girls in the 6-month delay condition performed at higher levels, relative to boys in the same condition (M = 1.41 and 1.07, respectively). However, the effect was only apparent on the events that the children were permitted to imitate. On events that they had only watched, girls and boys produced comparable numbers of individual target actions (M-- 1.11 and 1.02, respectively). The only other gender effect was in production of ordered pairs of actions. Although the performance of the 13-month-olds was not affected by gender, among the 16-month-olds, girls produced fewer correctly ordered pairs of actions, relative to boys (M = 0.62 and 0.86, respectively). Again, as was the case for the children tested on four-step event sequences, when verbal reminders were provided, no gender-related differences in performance were observed. In summary, although one cannot prove the null hypothesis, the relative paucity of reports of gender differences in the literature on early recall memory, coupled with the lack of meaningful effects in a large-scale study, implies that gender is not a major source of variability in early recall memory performance. There are, however, two exceptions to this general statement. First, gender effects seem to be apparent at times of "transition." They were apparent at the "dawning" of the ability to engage in long-term temporally ordered recall (Bauer et al., 2001; Carver & Bauer, 1999) and at the dawning of the ability to accurately reproduce lengthy, arbitrarily ordered sequences (Bauer, Hertsgaard, et al., 1998). Second, in a large-scale study, gender effects were apparent under conditions of high cognitive demand (i.e., recall after a long delay) and lesser contextual support (i.e., in the absence of verbal reminders; Bauer et al., 2000). B. EFFECTSOF CHILDREN'SPRODUCTIVEAND RECEPTIVELANGUAGE Despite the fact that elicited- and deferred-imitation tasks are nonverbal, children's own language facility might influence performance in several ways. First, in many studies (including all published studies from our laboratory), demonstration of to-be-remembered events is accompanied by verbal narration. Children's abilities to comprehend the language spoken to them is thus one potential source of variability. Second, even when narration is not provided (e.g., Meltzoff, 1985, 1988a, 1988b, 1995), language development is a source of variability because children with greater language skills potentially could verbally encode the events that they observe, thus providing themselves with additional retrieval cues. Indeed, the

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ability to "augment" nonverbal representations with language plays a role in the later verbal accessibility of memories likely encoded without the benefit of language (Bauer, Kroupina, et al., 1998; Bauer, Wenner, et al., 2002; Bauer & Wewerka, 1995, 1997). Nevertheless, systematic variability attributable to differences in children's language proficiency has been examined only in very specific, limited contexts. For example, in the Bauer, Hertsgaard, et al. (1998) study, we found that total productive vocabulary and early grammatical development were correlated with 28-month-old children's performance on lengthy (five-step) arbitrarily ordered sequences. The language variables were not correlated with performance on lengthy sequences constrained by enabling relations nor with performance on shorter (three-step) sequences. To our knowledge, language development has not been examined for potentially more general relations with memory performance. The Bauer et al. (2000) study affords the opportunity to examine the possibility that variability in language development accounts for systematic variance in longterm recall. Specifically, to match the children assigned to the different delay conditions (1-12 months), we used children's scores on the MacArthur Communicative Development Inventory for Toddlers (20-month-olds) and the MacArthur Communicative Development Inventory for Infants (13- and 16-month-olds; Fenson et al., 1994). Only vocabulary production data were available for the 20-month-olds, but data on both productive and receptive vocabulary were available for the 13- and 16-month-olds. We had completed MacArthur inventories for 93% of the children (336 of the 360 children in the sample). Children's reported productive vocabulary scores ranged from 0 to 651 words (across the 13- to 20-month age range); their reported receptive vocabulary scores ranged from 11 to 393 words (across the 13- to 16-month age range). The sample thus featured ample power and ample variability to permit detection of systematic relations between children's language and children's recall memory performance. To examine relations between vocabulary and recall across delay conditions (and thus take advantage of the large samples available), we converted children's mnemonic performance into z-scores. We then calculated correlations between children's recall and their reported vocabularies. For reported productive vocabulary, the correlations between language and memory performance ranged from -.02 to .15. None of the correlations was statistically reliable. For reported receptive vocabulary, the correlations ranged from -.09 to .23. The correlation of .23 between reported receptive vocabulary and delayed-recall of individual actions by 16-month-olds tested on four-step sequences approached significance (p < .07). None of the other correlations approached significance (p > .40). Thus, as measured in this large sample of children, neither reported productive nor receptive vocabulary was reliably related to recall memory performance. In summary, despite conceptual reasons for expecting relations between children's language facility and recall memory performance, virtually no relations

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obtained, even in a large sample with ample variability. The picture changes only slightly as children get older. For example, Gordon et al. ( 1 9 9 3 ) found that the language abilities of 5-year-olds were related to the amount of detail they reported for physical examinations at the doctor's office. However, in a comparable study, including both 3- and 5-year-old children, Greenhoot, Ornstein, Gordon, and Baker-Ward (1999) found no relations between language and either verbal report or physical enactment of the examination. Thus, although language ability might logically be related to recall memory in young children, compelling relations have not been observed. c. EFFECTSOF INDIVIDUALVARIABILITYIN INITIALLEARNING Individual differences in initial learning or encoding of to-be-remembered events are another possible source of variability in long-term recall. Retrieval of information about a previously experienced event depends on what was originally encoded about the event. Indeed, this fact presents a challenge to valid research on developmental differences in the length of time over which memory persists (Howe, 2000): Because older children learn at a faster rate than younger children, older children likely have more detailed and elaborated memory representations, relative to younger children. In the present context, we focus attention not on age-related differences in initial learning, but on individual differences therein as a possible source of individual differences in recall. Factors as fundamental as the speed of processing (e.g., Kail & Salthouse, 1994) will affect how much event-related information is encoded, as well as how effectively and efficiently information is consolidated for long-term storage and subsequent retrieval (see Bauer, Cheatham, Cary, & Van Abbema, 2002; Bauer, Van Abbema, & de Haan, 1999, for discussion). Until more is known about the processes of consolidation of to-be-remembered information for long-term storage, and possible individual differences therein, a potentially large source of variance in early long-term recall will remain unexplored. Moreover, identification of individual differences in initial learning as a source of variance in long-term recall only begs the question of the source of individual differences in initial learning. D. SUMMARYOF THREE SOURCESOF VARIABILITYIN EARLY RECALL MEMORY The study of variability in early recall memory does not have a long history. Two of the "first guess" possibilities for systematic variability in performance, namely, gender and language, have been rather disappointing as potential explanatory sources. Individual studies rarely include tests of possible effects associated with these factors. When possible effects are tested, they are not often found. Even in a large-scale study with sufficient power to detect variance associated with children's gender and vocabulary, effects were few. In the case of children's gender,

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although some isolated effects were detected, they disappeared under conditions of greater contextual support. In the case of children's language, no systematic relations were observed. Whereas a third potential source of individual differences has been identified, namely, variability in initial learning of events, its effects rarely have been acknowledged explicitly. Moreover, detection of significant variance associated with differences in initial learning would only change the focus of the question from the source (or sources) of individual differences in long-term recall to the source (or sources) of individual differences in initial mastery. Regardless of the specific focus of the question, significant variance remains to be explained. In the Bauer et al. (2000) study, we were able to quantify the amount. We investigated the variance accounted for by a combination of nine factors, including the child-related variables of age at exposure to the test events (13, 16, or 20 months), gender, and productive vocabulary (ranging from 0 to 651 words); features of the research design, including the number of steps in the tobe-remembered event sequences (three or four steps), the experimenter by whom the child was tested (two different experimenters conducted the sessions; a given child was tested by the same experimenter at each session), and the length of the delay between exposure and test (1, 3, 6, 9, or 12 months); and measures of children's performance in the imitation context, including baseline levels of performance, levels of initial learning of the events (as measured by immediate imitation of half of the sequences), and levels of performance on the new control events. Together, the nine variables accounted for 32% of the variance in children's recall of the individual target actions of the sequences; they accounted for 26% of the variance in children's recall of temporal order. Clearly, substantial variance remains to be explained. Accordingly, we look beyond children's gender, language, and initial learning to other characteristics of the developing child that in research with both younger infants and older children have been shown to influence memory performance, namely, temperament characteristics.

IV. Individual Differences in Long-Term Recall: Children's Temperament Characteristics A. WHATIS TEMPERAMENT? Temperament or behavioral style refers to constitutionally based patterns of responding to environmental stimuli (e.g., Gunnar, 1990; Rothbart, Ahadi, & Hershey, 1994; Rothbart & Bates, 1998). Many theories and operationalizations of the spectrum of behavioral patterns exist, yet most stem from the nine dimensions originally outlined by Thomas and Chess (1977). Moreover, three psychobiological dimensions underlie each theory or scale of temperament: positive affectivity or approach (e.g., response to an attractive toy), negative affectivity and inhibition (e.g., response to fear-provoking stimuli), and attention (i.e., orienting, regulation

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and control; for more detailed discussions, see Goldsmith, 1996; Gunnar & Nelson, 1994; and Rothbart, Derryberry, & Posner, 1994). B. WHY MIGHT TEMPERAMENT CHARACTERISTICS BE RELATED TO EARLY RECALL MEMORY?

One major motivation for examination of possible relations between early recall memory and temperament characteristics stems from behavioral observations. For example, encoding of a stimulus should be facilitated by attention to it. This fact is implicitly recognized in studies of infant memory. In research using visual recognition paradigms, researchers use "flashy" orienting stimuli (e.g., blinking lights) to ensure adequate attention prior to the display of to-be-encoded stimuli; as trials progress, testers tap on the screen in attempts to reengage infants' attention. In the context of research using elicited and deferred imitation, researchers demonstrate individual actions and sequences two or more times in succession to ensure that infants and young children, whose attention may "wander" during demonstration, be given adequate encoding opportunities. Given the significance of encoding to subsequent recall, young children's performance might well differ as a function of temperament characteristics such as regulation and control of attention. A second major motivation for examining relations between temperament and memory is that in the case of the temperament aspect of attention, in particular, they rely on a shared neural substrate (see C. A. Nelson & Dukette, 1998, for discussion). In both cases, the systems seem to consolidate and stabilize over the course of the latter part of the 1st year and throughout the 2nd year of life. The neural substrate implicated in recall memory was reviewed earlier (see section I.B). Behavioral expression of positive affectivity, negative affectivity, and attention is associated with activity in the limbic system, specifically the amygdala and hippocampus, and the parietal and frontal regions (Derryberry & Reed, 1996; Derryberry & Rothbart, 1997; Rothbart & Posner, 2001). This dimension of temperament becomes reliably functional by 6 months, when infants begin to smile, laugh, and approach rewarding stimuli (Derryberry & Rothbart, 1997; Rothbart & Bates, 1998). By 9 months, infants begin to evidence fearful behaviors linked with negative affectivity and inhibition. Such behaviors are supported by hippocampal circuits that respond to biologically prepared fear stimuli, punishment, and nonreward signals. Patterns of negative affectivity and inhibition stabilize over the course of the 2nd year, presumably in concert with developments in the associated neural circuitry (Derryberry & Reed, 1996; Derryberry & Rothbart, 1997). The third psychobiological dimension of temperament--attention--has been described as relying on two relatively independent neural systems (Posner, Petersen, Fox, & Raichle, 1988), the more anterior of which overlaps substantially with the network thought to subserve recall memory (C. A. Nelson & Dukette, 1998). The first system, the posterior attention network, is implicated in orienting, which

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involves the disengagement and shifting of attention from one stimulus to another. The neural structures involved in this network include the pulvinar, parietal cortex, and superior colliculus (Posner et al., 1988). Consistent with evidence of relatively early metabolic activity in these structures (Chugani, 1994), posterior attention network function is evident by 2 to 4 months of age. The second system, the anterior attention network, is implicated in sustained attention. The anterior network involves interconnections between the limbic system (especially the hippocampus) and the frontal cortex. Presumably as a function of the later development of some of the structures involved in this network (e.g., dentate gyrus of the hippocampus, prefrontal cortex), evidence of its function is not apparent until late in the 1st year and early in the 2nd year of life (e.g., Vecera, Rothbart, & Posner, 1991). Moreover, the ability to sustain and control attention changes considerably throughout the course of the 2nd year and beyond (Derryberry & Rothbart, 1997; Rothbart & Posner, 2001). c. EVIDENCEOF RELATIONSBETWEENTEMPERAMENT CHARACTERISTICSAND MEMORYIN INFANCYAND THE PRESCHOOLYEARS Although the logic of behavioral links between memory and aspects of temperament is compelling, there are few empirical tests of possible relations. Wachs, Morrow, and Slabach (1990) examined relations between parent-reported temperament (assessed via the Revised Infant Temperament Questionnaire: Carey & McDevitt, 1978), home environment (assessed via the Purdue Home Stimulation Inventory), and recognition memory (assessed via novelty preference) in 3-month-olds. Initial analyses revealed relations between home environment and memory performance. However, the relations seemingly were mediated by infants' temperament characteristics: When temperament scores were partialled out of the correlations, half of the observed relations between home environment and recognition memory fell below significance. Relations between infant temperament and visual recognition memory (measured via event-related potential, ERPs) also were reported by Gunnar and Nelson (1994). They found that 12-month-old infants whose parents reported them to express more positive affect showed more ERP activity during the visual recognition task. This finding suggests that the biological components of memory and positive affect are associated. Aspects of temperament also have been found to relate to verbal mnemonic expression by 3- and 5-year-old children. Specifically, Gordon et al. (1993) found that the parent-reported temperament characteristic of approach (assessed via the Temperament Assessment Battery for Children, TABC; Martin, 1988) predicted 3-year-olds' correct responses to open-ended questions, whereas greater negative emotionality predicted overall correct recall for 5-year-olds. In a separate study, Greenhoot et al. (1999) examined the impact of parent-reported temperament

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(assessed via the TABC) on memory suggestibility in 3-year-olds. Low persistence was associated with adoption of a "yes" bias during the suggestibility portion of the study, and high manageability was associated with more memory errors. Finally, Geddie, Fradin, and Beer (2000) found that for 4- and 7-year-olds, adaptability was associated with better recall as well as with correct denial of misleading information. Two features of the existing literature on relations between memory and temperament are noteworthy. One is its very sparseness. The second is that there do not appear to be any examinations of relations between memory and temperament in children between 12 months and 3 years. This is an unfortunate omission, given the magnitude of changes that occur during this time period. Despite the absence of research, we can generate some predictions about specific relations in this space of time. For example, if memory is assessed in a novel laboratory context, children who characteristically experience a greater degree of fear or inhibition might be less likely to engage in the task and thus not perform as well. Similarly, children who are highly active might find it difficult to remain on task and as a result also not perform as well. In contrast, children who are high in positive affect and approach, or children who are able to regulate and sustain their attention, or both, might more readily engage in the task, and, as a result, evidence better memory. Given that the temperament dimensions of positive and negative affectivity stabilize before the dimension of attention, we might further predict that emotionality would be related to memory performance at a younger age (such as that observed in Gunnar & Nelson, 1994), whereas attention would be predictive only later in development. D. RELATIONSBETWEENTEMPERAMENTAND MEMORYIN THE SECOND YEAR: AGE-RELATEDDIFFERENCES The Bauer e t al. (2000) study provides an opportunity to address possible relations between temperament and memory in children 13-20 months of age at the time of experience of the to-be-remembered events. Although the study was not designed to examine relations between temperament and memory, in the spirit of a pilot study, we obtained reports of temperament from some of the children's parents. We used the Toddler Behavior Assessment Questionnaire (TBAQ; Goldsmith, 1996), a parent-report instrument consisting of 111 items referring to a variety of scenarios describing situations in which young children might be engaged. An example question is, "When your child wanted to eat something sweet before dinner was finished but did not get it, how often did s/he protest by crying loudly?" Parents were asked to indicate how often their children had such a reaction within the last month (scales range from 1, "never," to 7, "always," with the opportunity to indicate that the situation does not apply). The 111 items on the questionnaire cluster into five independent subscales: Activity Level, Anger Proneness, Interest and Persistence, Pleasure, and Social Fearfulness.

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Completed instruments are available for twenty-eight 20-month-olds and twenty-five 13-month-olds. We calculated correlations between children's observed long-term recall and their scores on each of the five subscales of the TBAQ (Kleinknecht, Rademacher, & Bauer, 2002). No significant correlations were observed between recall and scores on the Anger Proneness and Social Fearfulness subscales. However, as shown in Table III, relations between parent-reported temperament characteristics on the other three subscales and recall performance were observed; different patterns of relations obtained for the 13- and 20-month-olds. Among the 13-month-olds, children's long-term recall was positively correlated with the Pleasure subscale of the TBAQ. The relation between the Pleasure subscale and long-term ordered recall was statistically significant, whereas that with production of the individual actions of the sequences was a trend (p < . 10). In other words, higher levels of recall, and in particular ordered recall, were observed for children whose parents indicated that, for example, (a) when meeting other children in the park or playground, their children willingly joined the other children; (b) when seeing a familiar adult, their children expressed joy and babbled and talked; and (c) when making a new discovery, their children smiled and seemed pleased. Importantly, children's scores on the Pleasure subscale were not related to their performance on the new control sequences. Thus the positive relations between Pleasure and performance after the delay are indicative of an effect on recall, as opposed to a more general influence on performance on the imitation task.

TABLE III Relations Between Parent-Reported Temperament Characteristics and 13- and 20-Month-Old Children's Delayed Recall TBAQ Subscale

Age group/event type 13-month-olds (df= 23) Old sequences New sequences 20-month-olds (df= 26) Old sequences New sequences

Dependent measure

Pleasure

Activity level

Recall of actions Ordered recall Recall of actions Ordered recall

.34 .45* -.12 - . 11

.35 .25 .29 .19

Recall of actions Ordered recall Recall of actions Ordered recall

-.12 .07 .02 .08

-.42* -.34 -.33 -.08

Interest and persistence

- . 12 .05 .05 -.06 .48** .46"* -.07 .10

Note: *, p < .05; **, p < .01; df, degrees of freedom. Correlations are based on children's performances after the verbal reminder. Source: Kleinknecht, Rademacher, and Bauer (2002).

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Among the 20-month-olds, long-term recall was negatively correlated with the Activity Level subscale and positively correlated with the Interest and Persistence subscale of the TBAQ. In the case of Activity Level, the correlation was statistically significant on the variable of production of individual actions and a trend for production of pairs of actions in the target order (p < . 10). In the case of Interest and Persistence, the correlations were reliable for both dependent measures. Specifically, lower levels of recall were observed for children whose parents said that, for example, their children (a) run through the house; (b) climb on furniture; and (c) like to play games that involve running, banging, and dumping out toys. In contrast, more robust recall was observed among children whose parents said that, for example, (a) when playing alone, their children remain interested in toys for extended periods; (b) when looking at picture books alone, their children stay interested and do not get bored quickly; and (c) when playing with a detailed or complicated toy, their children explore the toy thoroughly. In the case of relations with the Activity Level subscale, similar patterns were observed on both old and new control sequences. In contrast, in the case of the Interest and Persistence subscale, the relations were specific to performance on the old sequences. Thus, children whose parents reported them to have characteristically high levels of activity did less well on the imitation task more generally, whereas children whose parents reported them to have characteristically high levels of interest and persistence showed higher levels of recall memory. That the patterns of relations were different at 13 and 20 months is noteworthy. At the younger age, high levels of recall, and in particular ordered recall, were observed among infants whose parents reported them to find pleasure in new experiences. Perhaps these infants "embrace" the novelty of the situation and the events, and their resulting focus of attention on the model afforded them better recall. A similar pattern was observed in the Bauer, Wiebe, et al. (2002) study, in which infants 9 months of age were tested for recall after a 1-month delay. As noted earlier (see section II.A.1), approximately half of the infants demonstrated ordered recall over the delay whereas half did not. To study possible correlates of this pattern of performance, we examined the infant version of the TBAQ, the Infant Behavior Questionnaire (IBQ). The group of infants who demonstrated ordered recall after 1 month differed from the group that did not on only one of the subscales, namely, Smiling and Laughter. The Smiling and Laughter subscale is the infant analog of the toddler Pleasure subscale. The consistent pattern of relations at 9 and 13 months indicates that for children who are just "breaking into" the recall memory system, being a "happy camper" is a benefit. Whereas pleasure in novel situations facilitates memory for young infants, by 20 months deriving pleasure from new experiences apparently no longer is sufficient to ensure high levels of recall. Instead, by 20 months, in order to remember, children need to be able to focus and regulate their attention to the specific properties of the materials and their manner of combination. Characteristically high

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levels of activity are not conducive to this "contemplative" attitude and thus were observed to be negatively related to successful mnemonic (and problem solving, as in the case of new events) performance (see Kleinknecht et al., 2002, for discussion). This pattern--positive relations with the Pleasure subscale at 13 months but positive relations with the Interest and Persistence subscale at 20 months--suggests that with development, Pleasure "merges into" Interest and Persistence. In other words, the pattern leads to the expectation that infants who score high on Pleasure at 13 months will at 20 months have high scores on Interest and Persistence. Because the Kleinknecht et al. (2002) study is based on cross-sectional data, we cannot use it to address this possibility. However, preliminary analyses of data from a longitudinal sample on whom we have temperament data at 13 and 24 months are consistent with this suggestion. Specifically, in this sample, high Pleasure scores at 13 months are correlated with high Interest and Persistence scores at 24 months (r = .44, p < .002). In contrast, scores on the Interest and Persistence subscale at 13 months are not significantly correlated with scores on the Pleasure subscale at 24 months (r = .17, ns). It is perhaps more than coincidence that over the period during which long-term recall ability seems to consolidate and become reliable, we see that the ability to sustain interest and focus attention becomes important for acquiring and remembering new material. As reviewed earlier (see section IV.B), the anterior attention network implicated in controlled attention (Rothbart & Posner, 2001) overlaps the temporal-cortical network implicated in long-term recall (see, for example, Carver & Bauer, 2001, for a review). In both cases, the functions are dependent on feedforward and feed-backward connections between the frontal lobes and other brain regions such as the hippocampus. Indeed, developments in the control of attention probably have direct implications for developments in mnemonic function, and vice versa. In summary, early in the development of typical infants and children, being able to engage with new objects and people and finding joy in the process is associated with better memory. By 20 months, pleasure in new experiences is not enough. Rather, being able to regulate activity level, to focus and control attention, and thereby to maintain interest facilitates performance. E. RELATIONSBETWEENTEMPERAMENTAND MEMORY: DIFFERENCESWITHINSUBJECTS The Kleinknecht et al. (2002) study provides insight into relations between recall performance and temperament throughout the 2nd year of life. Over this space of time, being able to sustain and focus attention and to regulate activity level apparently becomes increasingly important to successful mnemonic performance. The results of work by Burch and Bauer (2002) suggest that these characteristics also shift in significance within subjects, as task demands change. The major

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purpose of this study was to examine effects of a different source of individual differences on young children's recall, namely, variation in maternal language during initial learning and delayed-recall testing. The motivation for and findings of the study as they relate to that purpose are discussed later (see section V.C). At this point we outline the design of the study and describe relations between parent-reported temperament characteristics and recall memory as a function of task demands. 1. Study Design Typically in the elicited- or deferred-imitation paradigm, to-be-remembered events are modeled by an experimenter and children are encouraged to imitate. However, in the Burch and Bauer (2002) study, we were interested in examining possible effects of variability in maternal language on young children's recall. To do so, we had mothers, rather than experimenters, test the children. Sixteen mothers and their 24-month-old children participated. Mothers demonstrated the test sequences for their children and then elicited their children's imitation (i.e., immediate recall) of the sequences. One week later, the dyads returned to the laboratory and mothers tested their children's delayed recall. To examine effects of cognitive demand on maternal language and children's recall, we presented the dyads with sequences of three different lengths: four-step, five-step, and six-step. Based on previous related research, we expected four-step sequences to be relatively easy for the 24-month-olds (Bauer et al., 2000) and five-step sequences to be an optimal level of challenge (Bauer & Travis, 1993). We expected six-step sequences to represent a level of difficulty within or perhaps even beyond the 24-month-olds' "zone of proximal development," because not until 30 months do children typically perform at above-chance levels on six-step sequences (Bauer, Dow, Bittinger, & Wenner, 1998; Bauer & Fivush, 1992). In addition to data on children's immediate and 1-week delayed-recall performance, we collected TBAQs for the children. 2. Relations Between Child Temperament Characteristics and Recall On the relatively "easy for age" four-step sequences, higher scores on the Activity Level subscale of the TBAQ were related to higher levels of both immediate and delayed recall of the individual actions of the sequences ( r = .58 and .57, respectively; unless noted, alpha levels are p < .05). However, activity was not clearly related to memory, per se, because in the baseline phase, prior to modeling, children rated higher on Activity Level tended to produce more of the individual actions of the sequences spontaneously (r = .42, p = . 10) and the relation between Activity Level and spontaneous production of ordered pairs was reliable (r = .63). Thus, a characteristically high level of activity was perhaps beneficial to engagement in the imitation task and this carded over to recall performance when task demands were low. Consistent with this suggestion, on the five-step sequences (considered to represent an optimal level of challenge), Activity Level was related

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to immediate recall of the individual actions of the events (r = .50). The relation was not observed for ordered recall or for recall after the delay, both of which impose greater demands. Moreover, Activity Level was not related to performance on the most challenging six-step sequences. That positive relations with the Activity Level subscale were observed only when task demands were low accounts for the different patterns of relations between children's scores on this subscale and their performance in the present sample relative to the Kleinknecht et al. (2002) study, in which the relations were negative. In that study, the cognitive demand was significantly greater, in that 20-monthold children's recall of four-step sequences was tested after a minimum delay of 1 month. In the Burch and Bauer (2002) study, children 4 months older were tested for recall immediately and after a 1-week delay. Across studies, the findings suggest that a characteristically high level of activity may be beneficial to performance when task demands are low but may interfere with performance when greater demands are imposed. In the Kleinknecht et al. (2002) study, we observed that high scores on the Interest and Persistence subscale were associated with high levels of long-term recall among 20-month-olds. A similar pattern was observed by Burch and Bauer (2002) on the most challenging sequences. That is, on the six-step sequences, higher scores on the Interest and Persistence subscale were associated with higher levels of delayed recall of the temporal order of the sequences (r = .50). Scores on the Interest and Persistence subscale were unrelated to performance on the fourand five-step sequences. Thus, greater control over attention was associated with strong performance on the most challenging sequences. In addition to the changing pattern of relations with the Activity Level and Interest and Persistence subscales as a function of task difficulty, there also was a pattern of changing relations between the Pleasure subscale and recall performance. Whereas on the four-step sequences there were no relations with Pleasure, on the five-step sequences, children rated high on Pleasure remembered fewer of the individual actions of the sequences over the delay ( r = - . 5 5 ) ; they also tended to remember fewer ordered pairs of actions ( r = - . 4 2 , p--.11). On the six-step sequences, the correlation between high ratings on the Pleasure subscale and delayed recall of the temporal order of the sequences was reliable (r = -.50). This pattern of negative correlation between scores on the Pleasure subscale and recall by 24-month-olds of the more challenging sequences is in contrast to the pattern observed for 9- and 13-month-olds, for whom high scores on the Pleasure subscale are positively related to recall performance. Based only on the currently available data, there is not a ready interpretation for this difference in direction of relation. However, we offer an interpretation when we consider relations between maternal language behavior and children's memory performance and between maternal language behavior and children's temperament characteristics (see section V.C).

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E SUMMARY

Although there are conceptual reasons to expect that children's temperament might be related to memory performance, there have been few studies of possible relations. In the context of the Bauer et al. (2000) study, we examined relations between parent-reported temperament characteristics and recall memory in 13- and 20-month-olds. Among the 13-month-olds, delayed recall was positively related to children's scores on the Pleasure subscale of the TBAQ. In contrast, among the 20-month-olds, delayed recall was positively related to children's scores on the Interest and Persistence subscale and negatively related to scores on the Activity Level subscale. Whether these relations will increase the proportion of variance in children's long-term recall for which we are able to account is yet to be seen (we were able to account for a maximum of 32% of the variance in Bauer et al., 2000): With the small samples that are available, we are not able to test multifactor regression models. In addition, the patterns await tests for replication using parent-report instruments, as well as tests based on direct observations of children's behavior in situations considered to be diagnostic of temperament (e.g., using the Laboratory Temperament Assessment Battery: Goldsmith & Rothbart, 1996). Finally, in a separate sample, relations between children's mnemonic performance and parental reports of children's temperament varied as a function of the level of challenge of the to-be-recalled material. Most significantly, whereas scores on the Activity Level subscale were related to performance under less challenging conditions (i.e., simpler sequences, immediate recall, memory for individual target actions as opposed to ordered recall), scores on the Interest and Persistence subscale were related to performance under the most challenging conditions (i.e., six-step sequences). These findings highlight the importance of evaluation of relations across multiple contexts that make different demands.

V. Children's Temperament and Mothers' Language as Interacting Sources of Individual Differences in Long-Term Recall The results of the Kleinknecht et al. (2002) and Burch and Bauer (2002) studies indicate that children's characteristic reactions and behaviors are a systematic source of variance in their recall memory performance. In the Kleinknecht et al. study, the effects were observed in a carefully controlled experimental context: Children were tested by professional staff trained to administer test materials following a constrained standard protocol. In contrast, in the Burch and Bauer study, the context was less controlled because children were tested by their mothers-none of whom had any prior experience with the elicited-imitation paradigm. It is noteworthy that the patterns of relations between children's temperament and their recall memory performances converged under such different conditions.

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The less-well-controlled context of the Burch and Bauer study affords more than converging evidence for the results of Kleinknecht et al., however. Maternal administration of the test sequences provides the opportunity to examine relations between mother's behavior and children's mnemonic performance. In the literature on memory development in preschool-age and older children, maternal differences in verbal behavior in mnemonic contexts have predicted children's performance. In the section to follow we review this literature briefly. We then describe variability in maternal verbal behavior with 24-month-old children in the context of the elicited-imitation paradigm. Finally, we examine relations between maternal language, children's recall performance, and children's temperament characteristics. A. RELATIONSBETWEENMATERNALLANGUAGEAND OLDER CHILDREN'SMEMORYNARRATIVES The literature on preschool-age and older children reveals relations between variability in maternal language and children's autobiographical or personal memory narratives. In brief, researchers have observed two "styles" of conversation. Mothers who frequently engage in conversations about the past, provide rich descriptive information about previous experiences, and invite their children to "join in" on the construction of stories about the past are said to use an elaborative style. In contrast, mothers who provide fewer details about past experiences and instead pose specific questions to their children (e.g., "What was the name of the restaurant where we had breakfast?") are said to use a repetitive or low elaborative style. Maternal verbal stylistic differences have implications for children's autobiographical memory reports. Specifically, children of mothers whose language more closely approximates the elaborative style report more about past events than children of mothers whose language more closely resembles the repetitive style (e.g., Fivush & Fromhoff, 1988; Hudson, 1990; Tessler & Nelson, 1994). Relations between matemal language style and children's memory narratives are observed concurrently and over time. For example, Reese, Haden, and Fivush (1993) found that maternal use of a more elaborative style when children were 40 and 46 months of age facilitated children's independent narrative accounts at 58 and 70 months of age. B. MATERNALLANGUAGEVARIABILITYWITHYOUNGERCHILDREN IN THE CONTEXTOF ELICITEDIMITATION Whereas most of the work on relations between maternal language style and children's memory narratives has been conducted with older children, there has been some research with pre- and early verbal children. For example, Hudson (1990) reported effects of differential degrees of maternal verbal elaboration on 24- to 30-month-olds' participation in memory conversations (see also Farrant & Reese, 2000). In the Burch and Bauer (2002) study, we sought to expand this small

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literature into the context of the elicited-imitation paradigm, thereby forging a link between the literatures on verbal and nonverbal recall. We asked mothers to present to their 24-month-old children four-, five-, and six-step sequences and then to elicit their children's recall of the sequences immediately and after a 1-week delay. To prepare them for their task, mothers were exposed to a silent videotape on which the sequences were demonstrated. Beyond that they were to "talk naturally"-mothers were given no instructions regarding how to verbally communicate as they demonstrated and then tested their children's memories for the sequences. To examine variability in maternal verbal style, we coded each of the mothers' utterances into one of the seven mutually exclusive and exhaustive categories described in Table IV. The first five categories, namely, elaborations, repetitions, affirmations, negations, and off-task, are those used in studies of maternal style variability in older children (Reese et al., 1993). The last two categories, deflections and regulations, were added to capture interactions that occurred as a result of the context of the prop-supported elicited-imitation task. Because very few negations were observed, they will not be discussed; although regulations of children's behavior were not infrequent, there were few correlations with this category and for this reason, it will not be discussed; and because off-task utterances are not related to the task, they will not be discussed (see Burch & Bauer, 2002, for additional details). Mothers exhibited variability in their language both when teaching the sequences and when testing their 24-month-old children's memories. As illustrated in Table V, the number of utterances in each category varied considerably, as did the total TABLE IV Categories of Maternal Utterances Used with 24-Month-Old Children in the Context of Elicited Imitation

Elaborations: Utterances that served to introduce an object or event or that provided additional information related to an object or event previously introduced. For example, "look at the farm stuff," and "where does the horse go on the farm?"

Repetitions: Utterances that were a verbatim or gist repetition of a prior utterance. For example, asking "what do we do on the farm?" after having just said "do we do something on the farm?"

Affirmations: Utterances that served to affirm a child's behavior or previous utterance. For example, "good job," or "that's fight."

Negations: Utterances that served to deny a child's behavior or previous utterance. For example, "is that what we do next?"

Off-task: Utterances not directed to the task, including such things as conversation with the professional staff member directing the sessions and comments about activities outside the laboratory. Deflections: Utterances that served to hold a place in the conversation (without providing any new information) or that directed the turn to the child. For example, "that's a farm (elaboration), isn't it (deflection)," and "look at this (regulation)---wow! (deflection)." Regulations: Utterances the purpose of which seemingly was to direct the child's behavior or attention to the task. For example, "sit down," and "watch me."

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TABLE V Maternal Language Variability with 24-Month-Old Children in the Context of Elicited Imitationa Step length Four- step

Five-step

Six- step

Category of maternal utterance

M

(SD)

M

(SD)

M

(SD)

Elaborations

36.73

(10.86)

38.78

(9.32)

60.90

(22.42)

Repetitions

4.48

(3.76)

5.43

(3.97)

8.30

(5.19)

Affirmations

27.58

(14.54)

23.20

(12.80)

34.89

(18.57)

Deflections Total a M,

15.06

(5.05)

14.40

(7.04)

125.73

(36.90)

124.29

(26.19)

17.71

(8.03)

168.95 (41.70)

Mean; SD, standard deviation.

number of category tokens produced. The number of utterances also varied across sequences step lengths, such that across categories, mothers produced substantially more total category tokens on the six-step sequences relative to the four- and fivestep sequences, which did not differ from one another. Nevertheless, mothers' levels of production of the different utterance types were correlated across levels of difficulty of the events. For example, the number of elaborations used as mothers elicited their children's immediate recall of the four-step sequences was correlated with the number of elaborations used to elicit immediate recall of the five- and sixstep sequences (r = .80 and .58, respectively); the number of elaborations used to elicit recall of the five- and six-step sequences also was correlated (r = .85). In other words, although mothers varied in the number of tokens produced across levels of difficulty of the task, they nevertheless exhibited stability across sequence lengths. Thus, even though in Burch and Bauer (2002) we used a nonverbal task with 24month-olds, we observed the same type of maternal language variability that has been reported in the literature on verbal narrative production with preschool-age and older children. C. RELATIONS BETWEEN MOTHERS' LANGUAGE, CHILDREN'S RECALL PERFORMANCE, AND CHILDREN'S TEMPERAMENT CHARACTERISTICS

Having observed variability in measures related to maternal style, we explored whether it was related to children's immediate or 1-week delayed recall performance, or to children's temperament characteristics, or to both. As will become apparent, the patterns of relations differed as a function of the level of difficulty of the to-be-remembered events. Because relations on the five-step sequences were a mixture of the patterns observed on the simpler and more difficult sequence lengths,

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for convenience, we present the relations for the four- and six-step sequence lengths only. We focus our discussion on the TBAQ subscales of Interest and Persistence, Pleasure, and Activity Level because, as observed in the Kleinknecht et al. (2002) study, they were the most predictive. Details on relations in the context of the fivestep sequences, and on the small number of relations involving the other subscales of the TBAQ, are available in Burch and Bauer (2002).

1. Four-Step Sequences On the relatively "easy for age" four-step sequences, maternal verbal behavior during the spontaneous, child-controlled baseline period was related to the total number of target actions that the children produced both immediately and after the delay. 3 Specifically, maternal use of elaborations ( r = .72), affirmations of children's behaviors (r = .62), and overall talkativeness (as measured by the total number of category tokens produced; r = .73) as children manipulated the eventrelated objects spontaneously all were correlated with the total number of target actions children produced in immediate recall (all p < .05). The same variables were correlated with the total number of target actions produced at delayed recall as well (r = .51, .85, and .79, respectively). Maternal verbal behavior was also related to children's engagement during immediate recall of the four-step sequences. Mothers who produced relatively more verbal elaborations during the immediate-recall period had children who produced both a larger number of total target actions and a larger number of different target actions (both r = .50). Production of a large number of total target actions by the children also was related to maternal repetition of utterances and to maternal affirmations (r -- .55 and .69, respectively). Maternal affirmations during the immediate-recall period also had a cross-lag relation to children's performance after the delay: Mothers who affirmed their children during immediate recall had children who were more engaged in the task at delayed recall, as measured by the total number of target actions produced (r = .58). The children also did their part to keep the system going: Children who produced a larger number of target actions during immediate recall had mothers who produced both a large number of affirmations and more total category tokens during delayed recall (r = .67 and .55, respectively). Finally, maternal affirmations during the delayed-recall period were related to production of individual target actions by the children (r = .64). 3 In the research discussed thus far, we have considered the number of different target actions that children produce as a measure of the exhaustiveness of recall and the number of pairs of actions produced in the target order as a measure of organization of the event representation. Early in our research on effects of maternal language on children's recall, we noted another dimension of difference, namely, variability in the sheer number of target actions (including repetitions) performed by the children on each trial. As discussed by Bauer (1993), we consider the total number of target actions produced, including repetitions, as a general indication of the children's engagement with the props and sequences.

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Two patterns in the data from the four-step sequences are especially noteworthy. First, on these easy sequences, maternal behavior was not in response to the children's memory performance, per se: Systematic variability in maternal language was apparent in the baseline phase, before any memory behavior was observed. Second, maternal verbal behavior was related to children's levels of engagement in the task (i.e., to the total number of individual target actions produced) but not to the exhaustiveness of children's recall or to its organization. That is, there was only a single correlation between maternal verbal behavior and the diversity of target actions that the children produced; there were no correlations with ordered reproduction of the sequences. These patterns suggest that on event sequences that are well within a child's developmental level, the mother-child interaction is not related to memory but rather to the dyad's established patterns of interaction in the context of new materials or experiences. Both mothers and children behavemthe one verbally and the other nonverbally; mothers who are more verbally engaged have children who are more nonverbally engaged. Examination of the pattern of relations between maternal verbal behavior and children's temperament characteristics revealed that mothers were especially verbally engaged with children rated as having high levels of Interest and Persistence. Specifically, in the context of the simplest sequences, mothers who rated their children high on the Interest and Persistence subscale of the TBAQ produced more elaborations (r = .58), more repetitions (r = .55), and more total category tokens ( r - - . 6 6 ) during the baseline phase. At immediate recall, they produced more repetitions (r = .52), more affirmations (r = .62), more elaborations (r = .49), and more total category tokens (r = .53). At delayed recall, they produced more affirmations (r = .63). Thus, children who were perceived as typically showing evidence of interest and persistence received in the elicited-imitation context more verbal scaffolding during baseline and immediate recall and, to a lesser extent, during delayed recall. Higher levels of maternal verbal scaffolding were in turn related to higher levels of engagement in the task. In the context of the four-step sequences, relations between maternal verbal behavior and children's temperament characteristics were confined to the Interest and Persistence subscale; maternal verbal behavior was unrelated to either the Pleasure or Activity Level subscales of the TBAQ. Consideration of the relations observed among maternal language, children's recall performance, and their temperament characteristics, suggests two "pathways" to high levels of recall in the context of a relatively easy task. The first route is the direct one taken by children whose parents rated them as having high levels of activity. As reviewed earlier (see section IV.E.2), children rated high on Activity Level performed well, both immediately and after the delay. Because variability in children's reported levels of activity was not related to variability in maternal language, we interpret the relation between Activity Level and recall as direct: Under low levels of challenge, a characteristically high level of activity may have beneficial effects on performance.

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The second route to a high level of performance is the indirect one afforded to children rated high in Interest and Persistence. Children whose parents perceived them as being high in Interest and Persistence were treated to high levels of maternal language during both baseline and recall. In turn, the children were highly engaged in the task, both immediately and after the delay. The association between levels of Interest and Persistence and children's performance is best characterized as indirect because scores on the Interest and Persistence subscale were not related to recall performance. Rather, the relations were between the temperament subscale and maternal behavior, and between maternal behavior and children's performance. We believe that this was observed because in the case of a relatively low level of challenge, such as that posed by four-step sequences, higher levels of selfgenerated and self-regulated attention are not necessary to ensure engagement in the task. Nevertheless, even in an "easy" context, a characteristically high level of attention is beneficial because of the high level of supportive maternal language that it engenders.

2. Six-Step Sequences In contrast to the four-step sequences, on the very difficult six-step sequences, maternal verbal behavior in the baseline phase was largely unrelated to children's behavior in the immediate recall period. The only relation observed was between total category tokens during baseline and children's engagement at immediate recall, as measured by the total number of target actions produced: r = .49, p = .05. The only other relations during the first session were between maternal verbal behavior and children's temperament characteristics. During baseline, mothers of children with low scores on the Pleasure subscale produced more elaborations (r = -.52); during modeling, they produced more deflections (r = -.50). Similarly, as they modeled the six-step sequences, mothers of children with lower scores on the Activity Level subscale produced more deflections (r = -.50) and more total category tokens (r = -.56). One interpretation of these patterns is that mothers whose children do not typically spontaneously show high levels of pleasure and activity saw greater need to engage their children in the more difficult sequences. The "ploy" was effective: Children whose mothers frequently deflected the turn to them as they modeled the event sequences during the first session produced a larger number of different target actions during delayed recall (r = .55). Whereas there were few concurrent relations between mothers' and children's behavior during the first session, there were a number of concurrent relations during the second session. Specifically, mothers who during delayed recall produced more elaborations, more repetitions, and more affirmations had children who were more engaged in the difficult task, as measured by the total number of target actions produced (r = .59, .54, and .80, respectively). Maternal verbal behaviors were delivered without regard for children's temperament characteristics. The only relation between children's temperament and maternal behavior at the second

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session was that mothers of children highly rated on the Interest and Persistence subscale provided more affirmations during delayed recall ( r = .55). The sparse pattern of correlation with temperament characteristics on the more challenging sequences relative to the simpler four-step sequences suggests that as the level of challenge increased, mothers responded more to the "situational" factor of their children's needs for more scaffolding and support and less to the "dispositional" factor of whether, under typical circumstances, their children show a high level of interest and persistence. Indeed, as noted earlier (see section V.B), across the sample, mothers provided more total category tokens on the more difficult six-step sequences, relative to the easier four- and five-step sequences. Nevertheless, as reviewed earlier (see section IV.E.2), children's scores on the Interest and Persistence subscale were related to levels of delayed recall on the six-step sequences. Thus, against a backdrop of uniformly high levels of maternal verbal scaffolding (i.e., provided regardless of children's temperament), children with characteristically higher levels of self-regulation of attention and interest had an advantage over their less self-regulated peers, perhaps because they were better able to capitalize on the scaffolding provided by their mothers. Finally, as noted earlier (see section IV.E.2), on the six-step sequences, children with high scores on the Pleasure subscale had lower levels of delayed recall. Why might a behavioral style characteristic that seemingly benefited recall at 9 and 13 months of age (Bauer, Wiebe, et al., 2002, and Kleinknecht et al., 2002, respectively) seemingly impair it at 24 months? We suggest that mothers of children rated high on Pleasure were not providing them with high levels of verbal scaffolding. Indeed, maternal language was more frequently directed toward children low on Pleasure than to children high on Pleasure. Children with characteristically high levels of engagement in novel situations may have been perceived as not needing assistance with getting or staying involved in the task and thus did not receive it. Although we do not mean to imply that mothers were consciously aware of the relation, we note that given the history of their children, mothers had reason to believe this to be true: At younger ages, high levels of Pleasure are associated with high levels of performance. In effect then, in the context of a difficult task, children with higher levels of positive affectivity were doubly disadvantaged: (1) their temperament characteristic of Pleasure did not afford them a direct benefit (the situation called instead for high Interest and Persistence), yet (2) their mothers did not seem to know that they might need assistance! D. SUMMARY

Variability in the ways in which parents talk with their preschool-age and older children has pronounced effects on the children's concurrent and subsequent autobiographical memory narratives (e.g., Reese et aL, 1993). With few exceptions (e.g., Farrant & Reese, 2000; Hudson, 1990), the impact of these differences in

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maternal style on younger children, and on other memory tasks (e.g., elicited imitation), has not been examined. In the Burch and Bauer (2002) study, we found that mothers of 24-month-old children differed substantially from one another in their verbal behavior as they engaged their children in event sequences of different levels of challenge; variability in maternal language was related to variability in children's performance. On the simplest sequences, mother--child interaction was not related to memory, per se, as much as it was to the dyad's established patterns of interaction in the context of new materials or experiences: Mothers who were more verbally engaged had children who were more nonverbally engaged. On the most difficult sequences, mothers seemed to vary their verbal behavior in response to the greater level of challenge imposed on their children. Suggestively, maternal verbal behavior also was related to children's temperament characteristics. Again the pattern differed as a function of level of difficulty of the task. On the simplest, four-step, sequences, mothers seemed to behave in accord with their perceptions of their children's abilities to regulate their own attention and interest. Children rated as higher on Interest and Persistence were treated to more verbal tokens than children rated as lower on this subscale. Maternal verbal "interventions" seemingly were not directed toward moderation of children's pleasure or active involvement in the task. Conversely, on the more challenging, six-step, sequences, there were few relations with the Interest and Persistence subscale and negative relations with the Pleasure and Activity Level subscales. One interpretation of this pattern is that mothers were responding to the greater demands of the more challenging sequences and providing more verbal tokens, without regard for their children's typical levels of interest. Similarly, on the more challenging sequences, they may have perceived the need for greater verbal scaffolding for children with characteristically lower levels of Pleasure and Activity Level. Although the Burch and Bauer (2002) results need to be replicated, they provide the first evidence of relations between maternal verbal behavior and 2-year-old children's temperament characteristics and thus represent an important step in understanding the interactions between characteristics of the child and characteristics of her or his parent.

VI. Conclusions and Implications The end of the 20th century marked the beginning of the study of developments in recall during the first 2 years of life. Since the initiation of inquiry, we have made great strides in describing mnemonic behavior and in identifying its determinants. For example, we have established that the capacity for long-term ordered recall is newly or recently emergent at about 9 months of age (Bauer et al., 2001; Carver & Bauer, 1999, 2001) and that it becomes both prevalent and more robust over the course of the 2nd year of life (Bauer et al., 2000). Moreover, contrary to what we might have predicted in the face of tenacious assumptions that early memory

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abilities were qualitatively different from later abilities (e.g., Piaget, 1952), without exception, factors that affect recall in older children and even in adults exert similar influences on recall memory in the first years of life (see, e.g., Bauer, 2002a, for a review). Because recall memory performance is multiply determined, we doubt the value in attempting to produce "growth chart" type functions of the lengths of time over which children of different ages will remember. Nevertheless, when we specify the conditions under which children will be tested (e.g., after a single experience or after multiple experiences, with or without verbal reminders), we can make sound predictions regarding the mean levels of performance of children of different ages. We have heretofore been less mindful of the variation about the mean, however. Limited attention on variability in early recall memory has been directed toward children's gender and their language proficiency. Individual studies, typically based on small samples, have revealed little evidence of individual differences associated with these potential sources of variance. The picture was not altered substantially when we analyzed for these sources of variability in the sample of 360 children who participated in the Bauer et al. (2000) study. Discussion of a third potential source of individual differences in long-term recall, namely, variability in initial learning, also proved disappointing, not because it accounts for little variance (on the contrary: see Bauer, Cheatham, et al., 2002) but because detection of the variance associated with differences in initial learning only shifts the focus of attention from the source of individual differences in long-term recall to the source of variability in initial learning. Children's gender and their vocabulary are plausible sources of variability in recall memory, but they were not perhaps the most "inspired" initial foci. In contrast, we suggest, on both behavioral and neurobiological grounds, examinations of relations between characteristics of children's temperaments and recall performance. Behaviorally, for example, a temperamentally inhibited child might, in a novel laboratory context, be less likely to engage in the tasks at hand. Conversely, children who are high in positive affect and approach, or children who regulate their own attention and interest, or both, might more readily engage in the task and as a result, evidence better memory. In addition, shared substrate serves as a basis for hypotheses concerning relations between children's temperament characteristics and their recall memory performance: The neural circuitry that supports recall memory overlaps with that implicated in regulation and control of emotion and attention. Although others have recognized this potentially rich soil (e.g., Greenhoot et aL, 1999; Gunnar & Nelson, 1994; Wachs et al., 1990), to our knowledge we are the first to work it in the age period during which both long-term recall ability and certain aspects of temperament (e.g., control of attention) consolidate and stabilize. In Figure 1 we provide a schematic representation of the observed relations. In the context of controlled laboratory tasks administered by professional staff who faithfully adhered to precise protocols, we found associations between temperament characteristics and memory. Specifically, 9-month-olds who were rated

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Temperament

characteristics

I

Haternal "~ verbal "~1 behavior

Neurodevelopment .,

Basic mnemonic ability

First Year

~. Second Year

Fig. 1. Schematic representation of relations between neurodevelopmental factors, temperament characteristics, and basic mnemonic abilities in infancy, and maternal verbal behavior at the end of the 2nd year of life. Note that not all observed or hypothesized relations are represented.

by their parents as having higher levels of positive affectivity evidenced higher levels of recall, relative to their same-age peers (Bauer, Wiebe, et al., 2002). Consistent with speculation that one source of the association between temperament and memory in this age period is shared neural substrate are findings of relations between measures of temperament and electrophysiological indices of brain activity (Gunnar & Nelson, 1994) and measures of recall memory and electrophysiological indices of recognition memory (Bauer, Wiebe, et al., 2002; Carver, Bauer, & Nelson, 2000). Thus, late in the 1st year of life, "cognitive" (memory) and "social" (temperament) measures are related to one another. We speculate that the relations are mediated by overlap in the neural substrates implicated in the respective domains. Throughout the 2nd year, there is a paucity of direct evidence of relations between changes in cognitive and social behavior and changes in brain. The lack of evidence is due in large part to 1- to 2-year-olds' intolerance of imaging procedures such as functional magnetic resonance or even electrophysiological recording. Nevertheless, as reviewed earlier, there is compelling logical evidence of relations between developments in brain and developments in both memory and temperament. There also is evidence of relations between temperament and memory. In the Kleinknecht et al. (2002) study, we reported that at 13 months of age, as they had been at 9 months, high levels of recall were related to dimensions of positive affectivity. At 20 months, high levels of recall were related to dimensions of attentional control. In the context of a laboratory task administered by 24-month-old

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children's mothers who were free to create their own protocols, high levels of recall were related to dimensions of activity under less challenging conditions and to dimensions of attentional control under more challenging conditions (Burch & Bauer, 2002). Together, these patterns could be taken to indicate that early in development, before infants and young children have much in the way of selfregulatory capacity, their characteristic reactions and affective responses influence their encoding and subsequent memory. As children develop the capacity to direct and control their own attentional resources, and thus to control their own actions and reactions, that capacity becomes a prominent determinant of mnemonic behavior, especially under conditions of cognitive challenge. The Burch and Bauer (2002) study, in which children's mothers served as their "experimenters," revealed that the dimensions of temperament that influence mnemonic performance differ as a function of task difficulty. It also provided insight on interactions between one source of individual differences in children's recall memory, namely, temperament, and another, namely, maternal verbal behavior. We found that, as it is in preschool-age and older children (e.g., Reese et al., 1993; Tessler & Nelson, 1994), variability in maternal verbal behavior was related to variability in 24-month-old children's recall. Specifically, mothers who used more elaborative language had children who were more engaged in the memory task. Maternal verbal behavior also was related to children's temperament characteristics. When memory demands were well within 24-month-old children's range of competence (i.e., on four-step sequences), mothers of children rated as high on the Interest and Persistence subscale used more elaborative language. As memory demands increased (i.e., on six-step event sequences), relations no longer were apparent. This suggests that temperament characteristics not only are direct sources of variance in children's recall, but also that they influence other factors that themselves contribute to variability in performance. Whether relations between measures of temperament and matemal verbal behavior in a mnemonic context would obtain in children earlier in the 2nd year of life is not known at this time. Perhaps because variations in children's temperament are more obvious or more ubiquitous than variations in mnemonic behavior, the mothers of the 24-monthold children in the Burch and Bauer (2002) study seemed to respond more to the "social" characteristic of their children's temperaments than to the "cognitive" characteristic of their children's mnemonic performance. That is, at the lower levels of challenge, maternal verbal behavior varied as a function of children's characteristic levels of attentional control. In contrast, regardless of the level of challenge, there were few relations between children's mnemonic performance in one phase of the protocol and maternal verbal behavior in the next phase of the protocol. This is consistent with reports from the autobiographical narrative literature with older children. In conversations with their preschool-age children, mothers exhibiting a more elaborative style behave "elaboratively" regardless of whether their children are contributing to the conversation (e.g., Fivush & Fromhoff, 1988). Indeed, one

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of the "hallmarks" of an elaborative style is that elaborations are offered, even of the most paltry contributions. What are the developmental implications of the web of relations between neurodevelopment, cognitive, and social influences on early memory? In the age of speculation that early mnemonic abilities were limited and qualitatively different from later abilities (e.g., Piaget, 1952), the relations would not necessarily have held interest beyond the bounds of infancy and very early childhood. However, in the age of recognition of essential continuities in mnemonic processes across wide developmental spans (e.g., Bauer, 2002a; Howe & Courage, 1993; Meltzoff, 1995), they compel consideration of how early patterns of association might relate to later autobiographical or personal narrative competence as well as later episodic memory and narrative ability more broadly. Indeed, we have grounds to speculate that the observed relations between characteristics of 2-year-old children and maternal verbal behavior in a mnemonic task have implications for later memory performance. In Figure 2 we provide a schematic illustration of just some of the relations that already have been observed. In section V.A, we mentioned existing evidence of relations between maternal verbal behavior and children's autobiographical memory narratives. Both concurrently and over time, maternal elaboration in joint mother-child conversational contexts is related to children's autobiographical memory contributions (e.g., Reese et al., 1993). Preliminary analyses of ongoing research in our laboratory suggest that maternal elaboration in the context of joint reminiscing also has implications for episodic memory more broadly as well as for general narrative conpetence. For example, mothers who use a more elaborative style in the context of reminiscing with their 6- to 9-year-old children have children who recall more items in a sort-recall task (E J. Bauer, M. M. Burch, M. Larkina, & A. Tian, work in progress). Maternal elaboration also is related to 6- to 9-year-old children's independent non-mnemonic narrative competence: Mothers who use a more "elaborative" questioning style have children who produce higher quality narratives in a story production task (Wenner, Lynch, Wilson, Bramhall, & Galbraith, 2002). Based on findings such as these and those reported in sections IV and V, we seem to be on relatively firm ground in speculating that, early in life, children who characteristically evidence the temperament characteristic of Interest and Persistence will be treated to a more elaborative maternal style. In turn, over the course of development, a more elaborative maternal style will be related to greater autobiographical narrative and episodic memory abilities as well as general narrative competence. We make two qualifications to this discussion. First, in this report we have focused on children's temperament characteristics as a source of systematic variability in mnemonic behavior. Temperament characteristics likely are not the only individual difference that eventually will prove to be related to the behavior of socializing agents such as parents and thus, directly or indirectly, to memory. As

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Children's language

Autobiographical memory

!

Temperament characteristics

Maternal verbal "J behavior behavi,,,

I Episodic memory

Neurodevelopment Narrative competence

Basic mnemonic ability

Infancy

~-

Early Childhood

Fig. 2. Schematic representation of relations between neurodevelopmental factors, temperament characteristics, and basic mnemonic abilities in infancy; and maternal verbal behavior at the end of the 2nd year of life; as well as relations between children's language competence in infancy and maternal verbal behavior at the end of the 2nd year; and maternal verbal behavior at the end of the 2nd year and subsequent developments in autobiographical memory, episodic memory, and general narrative competence in early childhood. Note that not all observed or hypothesized relations are represented.

suggested by inspection of Figure 2, another likely candidate source of influence is children's language. In section III.B, we discussed the lack of direct relations between expressive language (at 13, 16, and 20 months) and receptive language (at 13 and 16 months) and nonverbal expression of memory. However, just as there are indirect links between memory and maternal verbal behavior, via temperament, so might there be indirect links between memory and language, via maternal verbal behavior. Although not discussed in the current chapter, in the Burch and Bauer (2002) study, we found that children with higher reported productive vocabularies had mothers who were more elaborative in the context of presentation of the four-, five-, and six-step sequences. As we observed with temperament, the relations were strongest on the shortest event sequences, and as sequence length and thus challenge increased, the relations diminished. Again, we see that when material is within children's levels of competence, mothers seemingly respond to

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their children's "dispositions"; as material becomes more challenging, mothers seemingly respond to the situation, and provide more verbal support (regardless of their children's dispositions). It is important to note that even if, on more challenging material, maternal behavior does not differ as a function of children's characteristics, these characteristics still matter. In the Burch and Bauer (2002) study, we found correlations that suggested that children with high levels of Interest and Persistence were able to take advantage of the increases in verbal support that all of the mothers provided on the more challenging sequences, and they were able to parlay it into higher performance on the most difficult tasks. That is, there were relations between Interest and Persistence and maternal elaborations as well as relations between maternal elaborations and children's mnemonic performance. We may speculate that a similar pattern would be observed with children's language. That is, children with higher levels of language competence may be better positioned to take advantage of the increased verbal scaffolding provided by mothers on more difficult tasks (i.e., they would be able to understand more of what their mothers were saying). The second qualification that we make is that Figure 2 does not provide an exhaustive depiction of existing or hypothesized relations. Rather, Figure 2 represents the major links discussed in this report (largely on the left side of the figure) and some of their possible implications for later development (largely on the fight side of the figure); consistent with their prominent roles in this report, temperament characteristics and maternal verbal behavior are emphasized. Links representing (a) continuity in mnemonic function across the time frame and (b) between-domain relations in early childhood are just some of the elements missing from the figure. That there is continuity in mnemonic ability across the period of transition from infancy to early childhood, and even beyond, is suggested by data from long-term longitudinal research in our laboratory. The first link in the chain is that basic mnemonic abilities late in infancy, as measured in the elicited-imitation paradigm, are related to verbal expression of episodic memory at age 3 years (Bauer, Kroupina, et al., 1998). Second, at age 3 years, children's verbal expressions of episodic memory are related to their verbal contributions in the context of mother--child autobiographical reminiscing (Bauer & Burch, 2002). Third, children's contributions in the context of joint autobiographical reminiscing at age 3 years are related to independent autobiographical narratives at ages 6 to 9 years (Van Abbema, 2002). There also are relations between types of memory in early childhood. For example, preschoolers' autobiographical memory narrative production skills are related to independent measures of episodic memory task performance (Kleinknecht & Beike, 2002). It will be left to a future report to provide a more fully elaborated picture of the web of relations between and among mnemonic and extramnemonic contributors to adultlike memory performance across a variety of domains.

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Finally, throughout this chapter we have focused on normative trends and individual differences in typically developing infants and young children. We close by suggesting that in addition to affecting memory in typically developing children, individual differences in characteristics such as temperament and perhaps language competence might be related to the course of memory development in children from special populations. Specifically, they may function as "protective" factors in populations of children at risk for memory deficits (Bauer, 2001). At the close of the 20th century, most of the extant research on cognitive outcomes in at-risk populations concerned rather global measures of cognitive function, such as the Denver Developmental Scales and the Bayley Scales of Infant Development. To address the need for more fine-grained assessments, researchers began to use specific tasks, such as deferred imitation, to determine whether in special populations, deficits in mnemonic function are apparent. In one study of memory in a population potentially at risk, we used immediate and 10-min deferred imitation to examine mnemonic function in infants who had been born prematurely but who were otherwise healthy. Performance after the 10-min delay was correlated with gestational age at birth (i.e., infants born prematurely performed poorly on the task, relative to full-term infants), suggesting that deferred imitation may be especially sensitive to subtle differences in cognitive function associated with premature birth (de Haan, Bauer, Georgieff, & Nelson, 2000). Similarly, in Kroupina, Parker, Bruce, Gunnar, and Bauer (2000), we found that children who as infants had been adopted from international orphanages showed deficits on the 10-min deferred-imitation task, relative to matched homereared infants. Currently in progress are studies employing the 10-min deferredmemory task with infants born to diabetic mothers who, as a result, were prenatally iron deficient (C. A. Nelson, M. K. Georgieff, & P. J. Bauer, work in progress) and with infants who have been physically abused or neglected (D. Cicchetti, S. Toth, & P. J. Bauer, work in progress). In each of these cases, there is reason to believe that development of hippocampal structures may have been undermined, with resuiting deficits in explicit memory. If, in these samples, relations between infants' and children's temperament and maternal verbal behavior, or between language development and maternal verbal behavior, or both, obtain, then they might work to "protect" infants and young children from the worst of these deficits. In conclusion, in the last 2 decades of the 20th century, we documented general developmental trends in early recall memory. We discovered systematic interactions among factors that influence recall competence and performance in the first years of life, including task difficulty, children's temperament characteristics, and maternal language. Major tasks for the first decades of the 21 st century will be to elaborate these relations and to systematically explore other potential mediators and moderators of early recall memory development. This course will ensure continued progress in understanding normative developmental trends as well as the individual variability that heretofore has been largely hidden behind the mean.

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ACKNOWLEDGMENTS Support for the research reported in this chapter was provided by grants from the National Institutes of Health (HD-28425) to Patricia Bauer. Additional support for preparation of the manuscript was provided for Erica Kleinknecht through an Institutional National Research Service Award provided by the National Institutes of Mental Health (MH-15755) to the Institute of Child Development at the University of Minnesota. We also thank the many collaborators who have helped to make the research possible. Those most directly associated with the research discussed are Stephanie Bangston, Leslie Carver, Patricia Dropik, Christine Leehey, Jennifer Rademacher, Emily Stark, Jennie Waters, Jennifer Wenner, and Sandi Wewerka. We also thank Hill Goldsmith for consultation and the children and parents who graciously volunteered their time and energy to the efforts reported herein.

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INTERSENSORY REDUNDANCY

GUIDES EARLY

PERCEPTUAL AND COGNITIVE DEVELOPMENT

Lorraine E. Bahrick and Robert Lickliter DEPARTMENT OF PSYCHOLOGY FLORIDA INTERNATIONAL UNIVERSITY MIAMI, FLORIDA 33199

I. INTRODUCTION: HISTORICAL OVERVIEW AND PERSPECTIVES ON PERCEPTUAL D E V E L O P M E N T II. A M O D A L RELATIONS AND THE M U L T I M O D A L NATURE OF E A R L Y EXPERIENCE III. U N I M O D A L - M U L T I M O D A L D I C H O T O M Y IN D E V E L O P M E N T A L RESEARCH IV. N E U R A L AND B E H A V I O R A L EVIDENCE FOR INTERSENSORY INTERACTIONS V. INTERSENSORY R E D U N D A N C Y HYPOTHESIS: TOWARD AN INTEGRATED T H E O R Y OF P E R C E P T U A L D E V E L O P M E N T A. PREDICTIONS OF THE INTERSENSORY R E D U N D A N C Y HYPOTHESIS AND THE I M P O R T A N C E OF INCREASING SPECIFICITY IN E A R L Y DEVELOPMENT B. DIRECT EMPIRICAL SUPPORT FOR THE INTERSENSORY R E D U N D A N C Y HYPOTHESIS C. ON W H A T BASIS DOES INTERSENSORY R E D U N D A N C Y FACILITATE PERCEPTUAL DISCRIMINATION AND LEARNING? VI. S U M M A R Y AND DIRECTIONS FOR F U T U R E STUDY OF PERCEPTUAL D E V E L O P M E N T REFERENCES

I. Introduction: Historical Overview and Perspectives on Perceptual Development The world provides a richly structured, continuous flux of multimodal stimulation to all of our senses. Objects and events can be seen, heard, smelled, and 153 ADVANCESIN CHILDDEVELOPMENT AND BEHAVIOR,VOL.30

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felt, and as we move around and interact with the people, places, and objects in our environment we produce continuous changes in proprioceptive and visual feedback from our exploratory activities. Our senses provide overlapping and redundant information for objects and events in our environment. Dating as far back as Aristotle's De Anima and De Sensu, scientists have been intrigued and challenged by issues arising from the specificity of stimulation from the different senses and the nature of the overlap among them. How are objects and events experienced as unitary when they stimulate receptors that give rise to such different forms of information? How are disparate forms of stimulation bound together? Aristotle postulated a "sensus communis"--an amodal or common sense--which he thought was responsible for perceiving the qualities of stimulation that were general and not specific to single senses ("common sensibles"). According to Aristotle, common sensibles included motion, rest, number, form, magnitude, and unity. These properties are remarkably similar to those characterized as amodal by contemporary perceptual theorists (Bahrick & Pickens, 1994; J. J. Gibson, 1966, 1979; Marks, 1978; Stoffregen & Bardy, 2001). Centuries later, Locke (1690/1971) and Berkeley (1709/1910), among others, took a different approach to intersensory perception, proposing that perceivers must learn to interpret and integrate sensations before meaningful perception of objects and events could be possible. Following this "constructionist" approach, most modem theories of perception have been founded on the assumption that the different forms of stimulation from the various senses must be integrated or organized in the brain and therefore pose a "binding" problem for perception. It was thought that sensory stimulation had to be united by mechanisms that translate information from different codes and channels into a common language (Muller's "Law of Specific Energies," 1838). The constructionist view permeated thinking about the development of perception during most of the 20th century (Birch & Lefford, 1963; Friedes, 1974; Piaget, 1952), with most investigators assuming that we must learn to coordinate and integrate the separate senses. From this view, information had to be integrated across the senses through a gradual process of association across development in order to perceive unified objects and events. This integration was thought to occur by interacting with objects, experiencing concurrent feedback from different senses, and associating, assimilating, or calibrating one sense to another. For example, according to Piaget (1952, 1954) not until well into the first half-year following birth do vision and touch begin to be integrated. Through acting on objects, tactile feedback gradually endows the two-dimensional visual image of an object with three dimensionality. The attainment of perceptual abilities such as size and shape constancy, visually guided reaching, and object permanence were thought by Piaget and his colleagues (e.g., Piaget & Inhelder, 1967) to be slow to emerge and to depend on the gradual development of sensory integration. Prior to this integration, the visual world of the infant was thought to consist of images shrinking, expanding, changing shape, and disappearing and then reappearing capriciously.

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Until the activity-based achievement of sensory integration, infants were thought to perceive only unrelated patterns of visual, acoustic, or tactile stimulation, expressed by the well-known description of the world of the newborn infant by William James (1890) as a "blooming, buzzing confusion." Not until J. J. Gibson's (1966, 1979) seminal work on the "ecological" view of perception was the integration perspective on perceptual development seriously questioned. In a sharp break from traditional views, Gibson recognized that the existence of different forms of sensory stimulation was not a problem for the perception of unitary events but instead provided an important basis for it. He argued that the senses interact and work together to pick up invariant aspects of stimulation and should be considered as a "perceptual system." One important type of invariant information is amodal information that is common across the senses. As pointed out by Aristotle, amodal information is not specific to a particular sensory modality but is information common to several senses. Temporal and spatial aspects of stimulation are typically conveyed in multiple senses. As a case in point, the rhythm or rate of a ball bouncing can be conveyed visually or acoustically and is completely redundant across the two senses. The sight and sound of hands clapping likewise share temporal synchrony, a common tempo of action, and a common rhythm. We now know from a prolific body of research conducted over the last 25 years of the 20th century, inspired in large part by Gibson's ecological approach to perception, that even young infants are adept perceivers of arnodal stimulation (see Bahrick & Pickens, 1994; Lewkowicz, 2000; Lickliter & Bahrick, 2000; Walker-Andrews, 1997). Infants detect the temporal aspects of stimulation such as synchrony, rhythm, tempo, and prosody that unite visual and acoustic stimulation from single events, as well as spatial colocation of objects and their sound sources, and changes in intensity across the senses (see Lewkowicz & Lickliter, 1994, for a review). These competencies provide the foundation for the perception of meaningful and relevant aspects of stimulation in social and nonsocial events, and they are described in more detail in subsequent sections of this chapter. In our view, detection of amodal information in early development provides a radical and efficacious solution to the so-called "binding" problem (see J. J. Gibson, 1979). That is, detection of amodal information in early development does away with the notion of perceivers having to coordinate and put together separate and distinct sources of information. By detecting higher order information common to more than one sense modality, even relatively naive perceivers can explore a unitary multimodal event in a coordinated manner. The task of development becomes to differentiate increasingly more specific information from the global array through detecting invariant patterns of both multimodal and unimodal stimulation (E. J. Gibson, 1969; J. J. Gibson, 1979; Stoffregen & Bardy, 2001). Results from contemporary research on infant perception indicate that the fact that our senses provide overlapping and redundant information for certain properties of objects and events poses no problem for perception. Rather, as we

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argue later, this redundancy is a cornerstone of perceptual development, allowing optimal deployment of attention and the discovery of higher order perceptual structure. In this chapter we present a framework, the "intersensory redundancy hypothesis," that provides a way of conceptualizing the role of redundancy across the senses for promoting and organizing early perceptual and cognitive development. The intersensory redundancy hypothesis makes predictions about what aspects of stimulation will be attended to and processed more readily as a function of whether available stimulation for an object or event is multimodal or unimodal. Specifically, the intersensory redundancy hypothesis proposes that in early infancy information that is simultaneously available across two or more sensory modalities (amodal properties such as tempo, rhythm, and intensity) is highly salient and is therefore attended, learned, and remembered better than when the same information is presented in only one modality. Conversely, processing of some information is facilitated by unimodal stimulation. When modality-specific properties (such as pitch, color, pattern, or orientation) are presented in a single sensory modality, they are attended, processed, and remembered better than when the same properties are presented in the context of multimodal stimulation. We review a growing body of research that supports this framework and synthesize findings from human and animal as well as neural and behavioral studies that demonstrate the important role of intersensory redundancy in the development of perception, cognition, and communication.

II. Amodal Relations and the Multimodal Nature of Early Experience The young infant encounters a world of richly structured, changing, multimodal stimulation through his or her interactions with objects, events, people, places, and the self. This stimulation is experienced through a unified perceptual system that is sensitive to invariant aspects of stimulation across the senses (E. J. Gibson, 1969; E. J. Gibson & Pick, 2001). Several researchers have argued that amodal information can initially guide infant attention and perceptual learning in a manner that is economical, veridical, and adaptive (e.g., Bahrick, 1992, 1994, 2001; E. J. Gibson, 1969; E. J. Gibson & Pick, 2001; Walker-Andrews, 1997). As we have already described, amodal information is not specific to a particular sense modality but is redundant or invariant across two or more senses. Across the visual and tactile modalities, shape, texture and substance are amodal and specifiable in either modality. Any changes in intensity and temporal and spatial aspects of stimulation are amodal, including temporal synchrony, common rhythm, and tempo of action, which unite the movements and sounds of most audible and visible events. If the perceiver detects amodal information, then attention is, by definition, focused on a unitary, multimodal event. By detecting these higher order relations that encompass

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multiple forms of sensory stimulation, the problem of how infants come to integrate stimulation across the senses is effectively eliminated. For example, detection of amodal temporal synchrony, rhythm, and tempo may focus an infant's attention on the sights and sounds of a person speaking or on the visual and acoustic impacts of a bouncing ball. Consequently, the person or ball would be perceived as a unitary entity. Sensitivity to amodal relations can also act as a buffer against forming inappropriate associations across the senses, as infants would not readily relate the sounds of speech with other objects that do not share the temporal structure of the speech sounds. Researchers have demonstrated that infants perceive a variety of amodal relations across multiple senses (see Lewkowicz & Lickliter, 1994). For example, infants can perceive the relation between movements of a face and the sounds of a voice on the basis of temporal synchrony (Dodd, 1979), their common emotional expression (Walker, 1982; Walker-Andrews, 1997), and spectral information common to the shape of the mouth and a vowel sound (Kuhl and Meltzoff, 1984). Young infants can relate moving objects and their impact sounds on the basis of temporal synchrony (Bahrick, 1983, 1987, 1988; Lewkowicz, 1992, 1996), their common tempo of action (Bahrick, Flom, & Lickliter, 2002; Lewkowicz, 1985; Spelke, 1979), rhythm (Bahrick & Lickliter, 2000; Mendelson & Ferland, 1982), and collocation (Fenwick & Morrongiello, 1998; Morrongiello, Fenwick, & Nutley, 1998). They can also detect temporal information common to visual and acoustic stimulation specifying the substance and composition of moving objects (Bahrick, 1983, 1987, 1988, 1992) and the changing distance of moving objects (Pickens, 1994; Walker-Andrews & Lennon, 1985). In the area of visual-tactile perception, young infants can detect the common shape and substance of objects across vision and touch (E. J. Gibson & Walker, 1984; Hernandez-Reif & Bahrick, 2001; Meltzoff & Borton, 1979; Rose, 1994; Streri, 1993). Detection of these amodal relations guides selective attention and exploration of objects and events in the environment and promotes the perception of unitary multimodal events. Infants not only detect amodal relations, they also participate in temporally coordinated, co-regulated interactions with adult caretakers. Much early perceptual and cognitive development emerges in the context of close face-to-face interaction with caretakers. Adults regularly scaffold infants' attention and provide a rich interplay of concurrent visual, vocal, tactile, vestibular, and kinetic stimulation. The movements and vocal rhythms of infants have also been shown to contain a burstpause, turn-taking pattern that is intercoordinated with the temporal characteristics of adult communication (Jaffe, Beebe, Feldstein, Crown, & Jasnow, 2001; Sander, 1977; Stem, 1985; Trevarthan, 1993; Tronick, 1989). This sensitivity to temporal, spatial, and intensity information in human interaction promotes affective attunement between caregivers and young infants (Stem, 1985) and provides a vehicle for the development of intersubjectivity and shared perspective (Rochat & Striano, 1999; Trevarthan, 1993). Infants thus create, participate in, and respond to amodal

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information in their interactions with adult caretakers in a mutually co-regulated manner. This lays the foundation for later milestones of social and communicative functioning. Exploration of the self also provides one of the earliest and most potent and reliable sources of multimodal stimulation, as proprioceptive feedback always accompanies self-generated visual, vocal, and tactile stimulation (see Rochat, 1995). Infants engage in active, self-directed, intermodal exploration of their bodies (e.g., Butterworth & Hopkins, 1988; Rochat, 1993; Van de Meer, Van der Weel, & Lee, 1995) and the temporal and spatial contingencies between their movements and those of the multimodal objects and events in their environment (e.g., Bahrick, 1995; Bahrick & Watson, 1985; Rochat & Morgan, 1995; Schmuckler, 1995). In sum, a large body of converging evidence highlights the fact that infants are adept at perceiving, generating, and responding to a host of amodal relations uniting stimulation across visual, auditory, vestibular, tactile, and proprioceptive stimulation in the first months of life.

III. Unimodal-Multimodal Dichotomy in Developmental Research Despite the fact that the infant's world is inherently multimodal, and that virtually all perception, learning, memory, and social and emotional development emerges in this multimodal context, the majority of research in developmental psychology has focused on development in only a single sense modality at a time (see Kuhn & Siegler, 1998, for an overview of this type of research). This state of affairs likely resulted from the historical concern with sensory integration, the apparent intractability of the binding problem, and a lack of appreciation of the complex interdependencies among the senses. Scientists have traditionally specialized in unimodal areas such as vision, audition, or olfaction research, with subspecializations within each sensory area. As a result of this "unimodal" approach, the development of a specific skill or competence has been typically investigated detached from the rich multimodal context in which it occurs. For example, theories of speech and language development have typically focused on the unimodal speech stream, detached from the moving face and person that produce the speech. Research on infant memory and categorization has often focused on responsiveness to a unimodal visual display. Theories of face perception have been primarily based on studies of a unimodal, visual facial display devoid of movement and speech. Studies of the development of joint attention typically present the visual behavior detached from the auditory and tactile stimulation that typically co-occur (for further discussion, see Lickliter & Bahrick, 2001, and Walker-Andrews & Bahrick, 2001). The growth of the field of developmental psychology in general and the study of perceptual development in particular have tended to reflect this compartmentalization. Although research on the development of intersensory perception has

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grown more prominent, its theories and findings are for the most part segregated from research on the same questions explored with unimodal stimulation. Largely due to historical tradition, it has been placed alongside the other "senses" with "intersensory perception" constituting a separate area of inquiry (see Kellman & Arterberry, 1998, for an example). Thus, investigations of the development of a particular competence (e.g., aspects of memory, categorization, attention, speech perception) are likely to be conducted in separate studies of unimodal versus multimodal perception and to be undertaken by separate investigators. Consequently, research findings from the two areas are not well integrated and studies of unimodal and multimodal perception are difficult to compare, as they typically employ different methods and measures. Furthermore, few investigators actually compare responsiveness in one sense modality to responsiveness in two or more sensory modalities concurrently. Thus it is not known how perception of unimodal events such as the speech stream or moving faces generalizes to the multimodal world where speech occurs in the context of moving faces and vice versa. Importantly, research findings are consistent with the view that the senses interact in complex ways (e. g., King & Carlile, 1993; Lickliter, 2000; Lickliter & Hellewell, 1992; Stein & Meredith, 1993) and that different results are obtained when perception and cognition are investigated in the context of multimodal as compared with unimodal stimulation (Lickliter & Bahrick, 2001; Walker-Andrews & Bahrick, 2001). Research from the areas of unimodal and multimodal perception needs to be integrated if we are to develop a unified, ecologically relevant theory of the nature of perceptual development. Studying the single sensory system alone can, in many cases, result in a distortion of normally occurring patterns of sensory experience and consequently result in findings of limited generalizability. More studies are needed with both humans and animals that examine the development of skills and capabilities in a multimodal context and directly compare responsiveness to unimodal versus multimodal events in single research designs. Furthermore, just as unimodal and multimodal research is not well integrated, neither is behavioral research well integrated with research in the neurosciences. The proliferation of research on multisensory functioning in the neurosciences (see Calvert, Spence, & Stein, 2002) makes findings in this area of central importance to a biologically plausible theory of the development of intersensory perception.

IV. Neural and Behavioral Evidence for Intersensory Interactions Because the traditional view is that the different sensory modalities utilize separate and distinct neural pathways, neural "integration" of separate streams of sensory information has typically been viewed as necessary for adaptive perception and cognition. However, it has become increasingly clear that the separate

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senses are not so separate at the level of the nervous system (Knudsen & Brainard, 1995; Meredith & Stein, 1986; Stein, 1998; Stein & Meredith, 1993). This appreciation of the multimodal nature of the brain calls into question the long-standing view that higher order perceptual processing and cognition is needed to achieve successful binding or integration across the sensory modalities. Evidence obtained from neuroimaging studies reveals that many areas of the cortex and subcortex previously thought to receive input from only one sensory modality respond reliably to multisensory stimulation (see Calvert, 2001 a, for a review). Furthermore, a number of empirical investigations have shown that both young and mature animals have well-organized inputs from different sensory modalities converging on the same target structure in the brain (e.g., Frost, 1984; Innocenti & Clarke, 1984). This body of evidence from the neurosciences has led investigators to a growing appreciation of the brain's sensitivity to multimodal information (see Calvert et al., 2002; Stein & Meredith, 1993), but such an appreciation is not yet widely held by developmental psychologists and has yet to be incorporated into our thinking about the nature and direction of early perceptual organization. Here we briefly review some of the available neural evidence informing the study of perceptual and cognitive development. The most investigated site of multimodal convergence is the superior colliculus, a midbrain structure known to play a fundamental role in attentive and orientation behaviors (reviewed in Stein & Meredith, 1993). Multisensory neurons have been found in the superior colliculus of cats (Meredith & Stein, 1983), monkeys (Jay & Sparks, 1984), and several species of rodents (Wallace, Wilkenson, & Stein, 1996). The multisensory neurons in the superior colliculus respond to input from several sensory modalities and provide a neural substrate for enhancing responsiveness to stimuli that are spatially and temporally aligned. For example, in guinea pigs, visual experience is required for the normal elaboration of the sensory map of auditory space in the superior colliculus (Withington-Wray, B inns, & Keating, 1990). Guinea pigs reared in darkness fail to develop an auditory map, supporting the view that normal development of a map of auditory space requires the coincident activation of neural activity deriving from the convergence of both auditory and visual input arising from common stimuli. The activity-based alignment of different sensory maps in the brain and the responsiveness of these areas to intersensory convergence is likely a critical feature of multisensory perception (Stein & Meredith, 1993). Sites of multisensory convergence have also been reported at the cortical level of the brain in cats (Wallace, Stein, & Meredith, 1992), monkeys (Mistlin & Perrett, 1990), rats (Barth, Goldberg, Brett, & Di, 1995), and humans (Calvert, 2001 a; Giard & Peronnet, 1999), suggesting that the mammalian brain is inherently multimodal in structure and function. Of particular interest to theories of intersensory functioning is the finding from a number of neuroanatomical and neurophysiological studies indicating that the temporal and spatial pairing of stimuli from different sensory modalities can elicit a

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neural response that is greater than the sum of the neural responses to the unimodal components of stimulation considered separately (the so-called "multiplicative or superadditive effect" reviewed in Stein & Meredith, 1993; Stein, Meredith, & Wallace, 1994). In other words, the activity of a neuron exposed to multisensory stimulation (i.e., simultaneous auditory and visual stimulation) differs significantly from the activity of the same cell when exposed to stimulation in any single modality (Meredith & Stein, 1986). Spatially coordinated and synchronous multimodal stimulus combinations have been shown to produce significant increases over unimodal responses in several extracellular measures of neural activity, including response reliability, number of impulses evoked, and peak impulse frequency. This superadditive effect of bimodal stimulation, in which the magnitude of neural effects resulting from bimodal stimulation consistently exceeds the level predicted by adding together responsiveness to each single-modality stimulus alone (i.e., neural enhancement) has also been reported in behavioral investigations. For example, Stein, Meredith, Honeycutt, and Wade (1989) demonstrated that the effectiveness of a visual stimulus in eliciting attentive and orientation behaviors in cats is dramatically affected by the presence of a temporally congruent and spatially collocated stimulus in the auditory modality. These findings provide further support for the notion of differential responsiveness to unimodal versus multimodal stimulation and indicate that spatially and temporally coordinated multimodal stimulation is highly salient at the level of neural responsiveness. There is also compelling neurophysiological and behavioral evidence of strong intermodal linkages in newborns, young infants, and adults from a variety of species, including humans (e.g., Carlsen & Lickliter, 1999; King & Carlile, 1993; King & Palmer, 1985; Knudsen & Brainard, 1991, 1995; Lewkowicz & Turkewitz, 1981; Lickliter & Banker, 1994; Massaro, 1998; Mellon, Kraemer, & Spear, 1991; Withington-Wray et al., 1990). Experimental manipulations with animal subjects that augment or attenuate sensory stimulation in one modality consistently lead to significant effects on the development of perception in other sensory modalities and on the development of intersensory functioning during both the prenatal and postnatal periods (Lickliter & Banker, 1994; Lickliter & Hellewell, 1992; Radell & Gottlieb, 1992; Sleigh & Lickliter, 1997). For example, Lickliter and Lewkowicz (1995) showed the importance of prenatal tactile and vestibular stimulation to the successful emergence of species-typical auditory and visual responsiveness in bobwhite quail chicks. Hein and colleagues (Hein, 1980; Hein & Diamond, 1983; Held & Hein, 1963) demonstrated that visual stimulation provided by young kittens' own locomotion was necessary for the development of eye-paw coordination and visually guided behavior. Eye-paw coordination was found to develop normally in kittens allowed to simultaneously walk and look at objects and events, but did not develop normally when kittens could only look at things while being moved passively. Kittens denied visual feedback from locomotion also consistently failed to develop visually guided reaching. Of course, under normal developmental

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conditions, convergence between multimodal visual and proprioceptive stimulation is a regular aspect of postnatal experience and such convergence appears to be an experiential requirement of normal perceptual development. Studies of neural and behavioral development have thus revealed strong intermodal interactions in newborns and infants, with stimulation in one sensory modality influencing and even calibrating responsiveness in other modalities in an ongoing manner. Research with human adults has also provided compelling support for the salience of intersensory congruence. For example, Sathian (2000) found that the visual cortex can be involved in tactile perception in adult humans. In this study, PET scans of blindfolded subjects performing a tactile discrimination task (determining the orientation of ridges on a surface) revealed increased activity in the visual cortex. Furthermore, when function of the visual cortex was interfered with by means of transcranial magnetic stimulation, tactile perception was significantly impaired. In a similar vein, Calvert (2001b) scanned the brains of adults when they smelled odors, looked at colors, or did both simultaneously. Olfactory areas of the brain became particularly active when the colors and scents were congruent (i.e., a red strawberry) as compared with incongruent (i.e., a blue strawberry). Calvert concluded that multisensory congruence enhances neural responsiveness, whereas incongruence serves to suppress neural responsiveness (for a similar view, see Stein, 1998). The potent intersensory interactions present in early development appear to continue to affect perceptual responsiveness in adulthood. Several perceptual illusions also underscore the existence of intersensory convergence and its role in guiding attention and perceptual discrimination. The wellknown McGurk effect (McGurk & MacDonald, 1976), an auditory-visual illusion, illustrates how perceivers merge information for speech across the senses. When we view the face of a person speaking a speech sound such as "ga," while hearing a different speech sound, "ba," the perception is of another sound, "da," a blend between the concurrently presented auditory and visual stimulation. Infants also show evidence of this effect in the first half-year following birth (Rosenblum, Schmuckler, & Johnson, 1997), indicating that visual input has significant auditory consequences, even during early development. Auditory input has also been shown to have dramatic consequences for visual perception. Scheir, Lewkowicz, and Shimojo (2002) demonstrated an audiovisual "bounce" illusion in young infants. Without sound, adults perceive two disks to be moving horizontally and passing through one another on a computer screen (streaming). When a discrete sound is added at the point of contact between the disks, adults report that the two disks appear to bounce against one another and change direction of motion. Young infants also appear to perceive the addition of sound to change the nature of the visual display from streaming to bouncing, indicating convergence across the modalities and demonstrating that sound can alter the perception of a visual event even during infancy. Shams (2000) reported a similar intersensory illusion in which sound can make adults see visual illusions.

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Adults hearing two beeps while seeing one flash of light reported that they saw two flashes. Furthermore, neural activity in the visual cortex (thought to be specific to visual processing) was found to be essentially equivalent whether the participant actually saw two flashes (with no beeps) or just one flash accompanied by two beeps, suggesting that neural and behavioral consequences are operating in parallel. These examples drawn from neural and behavioral studies indicate that intersensory convergence is integral to perceptual functioning. Inputs from our separate senses interact and influence one another more than we have acknowledged or appreciated for much of the 20th century. Furthermore, this influence results in the perception of emergent properties of stimulation qualitatively different from the perception of input from the separate sensory modalities. In our view, theories of behavioral development must be informed by knowledge of neural development and responsiveness, and vice versa. Simply put, our psychological theories of intersensory functioning must be biologically plausible. That is, they must be consistent with available findings on intersensory convergence from the neural level of analysis, the physiological level of analysis, and with the complex intersensory interactions known to exist in the very early stages of perceptual processing.

V. Intersensory Redundancy Hypothesis: Toward an Integrated Theory of Perceptual Development Intersensory redundancy refers to a particular type of multimodal stimulation in which the same information is presented simultaneously and in a spatially coordinated manner to two or more sensory modalities. For the auditory-visual domain, it also entails the temporally synchronous alignment of the information in each modality. Only amodal properties (e.g., tempo, rhythm, intensity) can be specified redundantly because, by definition, amodal information is information that can be conveyed by more than one sense modality. Thus, the sights and sounds of hands clapping provide intersensory redundancy in that they are synchronous, collocated, and convey the same rhythm, tempo, and intensity patterns across vision and audition. As depicted in Figure 1, intersensory redundancy is best viewed as arising from an interaction between the organism and its environment. Redundancy is not a property of the structure of the organism (its nervous system and sensory systems), nor is it a property of the structure of objects and events in the environment. Rather, it results from an interaction between a structured organism and a structured environment. Redundancy is experienced when an active perceiver explores multimodal events with multiple coordinated senses. For example, one might explore a person speaking by looking and listening. In this case, the perceiver would experience redundantly specified information for the tempo, rhythm, and intensity patterns of auditory-visual speech. However, when the perceiver looks away

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Fig. 1. Intersensory redundancy results from the dynamic relationship between a structured organism and a structured environment. Redundancy arises from exploration by a nervous system specialized forpicking up different types of energy and their overlap andfrom unitary objects and events thatprovide a structured array ofmultimodal stimulation.

from the speaking person, or if the speaker leaves the room while talking, the perceiver no longer experiences redundantly specified information for tempo, rhythm, and intensity patterns. Rather, he or she perceives unimodal information for these speech properties. Thus, the perception of redundancy is dynamic in the sense that it can change from moment to moment as the relation between the nature of the organism's exploratory behavior changes and as the nature of the objects and events in the environment change. It is the convergence of information in two senses that makes amodal properties salient. Redundancy thus relies on both a nervous system specialized for different types of energy and the ability of the senses to provide overlapping information about objects and events that are unitary in the world. Redundancy is only apparent across different forms of stimulation and in this sense requires specific forms of energy from the different sensory modalities. As we describe in more detail in the sections that follow, intersensory redundancy is highly salient and can direct selective attention and facilitate perceptual learning in early development. In our view, intersensory redundancy is a particularly important and salient form of stimulation available to infants and plays a foundational role in early perceptual and cognitive development. Research with both animal and human infants indicates that different properties of stimuli are highlighted and attended to when redundant multimodal stimulation is made available to young organisms as compared with unimodal stimulation from the same events (see Bahrick, 2002; Bahrick & Lickliter, 2000; Lickliter & Bahrick, 2002). That is, young infants are especially adept at detecting amodal, redundant stimulation and detection of this information can organize early attention and provide a foundation for and guide and constrain perceptual development. We proposed an "intersensory redundancy hypothesis" to account for how this might be the case (Bahrick & Lickliter, 2000). The intersensory redundancy hypothesis describes how infants' attention will be allocated to different stimulus properties of objects and events as a function of the type of exploration (unimodal vs multimodal) afforded by the event. It also proposes consequences of this pattern of exploration for perception, learning, and memory.

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One tenet of the intersensory redundancy hypothesis holds that in early development, information presented redundantly and in temporal synchrony to two or more sense modalities recruits infant attention and facilitates perceptual differentiation of the redundant information more effectively than does the same information presented to one sense modality at a time. From this view, detection of higher order amodal relations in multimodal stimulation from an object or event causes amodal stimulus properties to become "foreground" and other properties of the object or event to become "background." Thus, intersensory redundancy affects attentional allocation and this in turn can promote earlier processing of redundantly specified properties of stimulation (temporal and spatial aspects) over other stimulus properties. Because intersensory redundancy is readily available in the multimodal stimulation provided by our environment and our interaction with it, perception, learning, and memory of amodal properties likely precedes that for other stimulus properties. This "amodal processing precedence" in turn, can have long-range effects on perception, cognition, and social and emotional development. Because all our fundamental human capabilities emerge and develop in a multimodal context, rich with intersensory redundancy, these initial conditions can continue to influence the trajectory and organization of development. And because sensitivity to intersensory redundancy is present early in development and redundancy is so pervasive, it can create a cascading effect across development such that its consequences manifest in an ever-widening trajectory in a variety of domains (see Michel & Moore, 1995, and Moore, 1990, for examples of cascading effects in development). However, not all exploration of the objects and events in our environment makes multimodal stimulation available. In fact, intersensory redundancy is often not available for a particular event, either because the perceiver is not actively exploring that particular event with multiple senses, or because the event that is the focus of attention is not providing redundant simulation at that moment in time to the senses through which the perceiver is exploring (e.g., the perceiver is just looking at, but not touching, a stationary or silently moving object). In this case, amodal information for the event may be unavailable or available only in a single sense modality. For example, one might experience the rhythmic sounds of speech from a neighboring room, or the sight of a light blinking at a regular rate on a nearby appliance. The amodal properties of rhythm and rate would then be specified unimodally rather than redundantly. In this case, the amodal information of rhythm and rate would not be salient and there should be no amodal processing precedence. When only unimodal stimulation is provided for a particular property, there is no competition from intersensory redundancy. Therefore, attention is more likely to be recruited toward modality-specific properties of stimulation. Modality-specific properties are qualities specific to a particular sense modality. For example, color and pattern can only be perceived visually and pitch and timbre can only be perceived acoustically. According to a second tenet of the intersensory redundancy hypothesis, when only unimodal stimulation is available for a particular property,

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attention to modality-specific properties should be facilitated relative to other stimulus properties. Thus, we hypothesize that unimodal exploration enhances perceptual differentiation of modality-specific and nonredundantly presented properties, as compared with the same properties presented in the context of multimodal, redundant stimulation (Bahrick & Lickliter, 2000). Optimal differentiation of visible qualities of an object or event should occur when there is no competition from auditory stimulation, which creates intersensory redundancy and recruits attention away from the visible qualities. For example, in early development, differentiation of the appearance of a person's face would be optimal when the individual was silent, differentiation of the nature of their particular voice would be optimal when their face was not visible, and differentiation of the prosody, rhythm, tempo, and timing of language would be optimal when viewing a speaking person. This observation is consistent with observations of the early emergence of sensitivity to the prosody in speech (Cooper & Aslin, 1989; Fernald, 1984). Thus, according to the intersensory redundancy hypothesis, the nature of exploration (unimodal vs multimodal) interacts with the type of property explored (amodal vs modality-specific) to determine the attentional salience and processing priority given to various properties of stimulation. As can be seen in Figure 2, bimodal exploration of amodal properties and unimodal exploration of modalityspecific properties receive priority in processing. In contrast, processing is relatively disadvantaged for bimodal exploration of modality-specific properties (e.g., listening to the pitch and timbre of a voice while also seeing the speaking face) and for unimodal exploration of amodal properties (e.g., seeing a rhythm displayed visually without sound, or hearing a rhythm in sound without visual

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Fig. 2. Predictions of the intersensory redundancy hypothesis. Facilitation versus attenuation of attention and perceptual processing for amodal versus modality-specific properties of stimulation as a function of the type of stimulation (multimodal or unimodal) available for exploration.

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accompaniment). Perceptual development is thus characterized by an intercoordination between exploration of amodal and modality-specific properties of events, and because of the prevalence of redundancy, detection of amodal information typically leads and constrains learning about modality-specific properties of stimulation. The intersensory redundancy hypothesis makes predictions about multimodal and unimodal processing of objects and events. This is unusual in that predictions about and attempts to synthesize research from these areas that have previously been studied separately are uncommon. As we discussed in section III, the perception of multimodal and unimodal aspects of stimulation have typically not been studied together, making comparisons across studies that use different methods and stimuli difficult. The intersensory redundancy hypothesis promotes research investigating unimodal and bimodal perception of various properties of events in single designs. Further, it makes a priori predictions about the effects of different types of stimulation on perception of different properties and fosters comparisons across domains that have typically been segregated. This model of selective attention is simplistic in the sense that it is based on the assumption that other contributing factors are held relatively constant. We acknowledge that factors such as goals, expectations, the intensity and amount of stimulation, and organismic factors (hunger, fatigue, arousal) also influence attention. The intersensory redundancy hypothesis is designed for the purpose of making predictions about which aspects of stimulation will be attended and processed as a function of whether the attended stimulation is unimodal or multimodal. A. PREDICTIONSOF THE INTERSENSORYREDUNDANCY HYPOTHESISAND THE IMPORTANCEOF INCREASINGSPECIFICITY IN EARLYDEVELOPMENT The intersensory redundancy hypothesis grew out of a synthesis of the neural, comparative, and human infant research on intersensory perception. Consequently, several fundamental aspects of the hypothesis are supported by existing research findings. Of particular importance, the prediction of amodal processing precedence is consistent with E. J. Gibson's (1969) differentiation view of perceptual development and with prior infant perception research showing that perceptual development proceeds in order of increasing specificity in early infancy. Bahrick (1992, 1994, 2001) demonstrated that global amodal relations (e.g., temporal synchrony between the sights and sounds of object impact) were detected developmentally prior to nested amodal relations (temporal microstructure specifying the composition of objects striking a surface), and this amodal information was detected prior to modality-specific aspects of the events (the color/shape of the object and the pitch of its impact sound). Detection of temporal synchrony was evident at even the youngest ages tested, whereas detection of nested composition information did not emerge until 7 weeks of age and increased dramatically across age. Detection

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of modality-specific pitch--color/shape relations in the same events did not emerge until 30 weeks of age. Early detection of temporal synchrony is important because synchrony can specify the unitary nature of the audible and visible stimulation. Once infants attend to unitary audiovisual events, differentiation of other properties can proceed in a veridical manner. In contrast, if infants learned about arbitrary audiovisual relations prior to detecting amodal relations, generalization would not be appropriately constrained (e.g., all red objects do not produce high pitched, jingling sounds). Thus, detection of properties in order of increasing specificity acts as a buffer against learning inappropriate audiovisual relations (Bahrick, 2001). Researchers have demonstrated that early detection of amodal relations can indeed guide and constrain perceptual learning about more specific arbitrary relations. For example, 7-month-old infants can learn the arbitrary relation between the appearance of an object and the speech sound paired with it when the object moves in synchrony with the sound, but not when amodal synchrony is absent (Gogate & Bahrick, 1998). Six-month-olds also learn to relate the color and pattern of an object and its tactually experienced shape in the presence of amodal shape information (available simultaneously to touch and sight) but not in its absence (HernandezReif & Bahrick, 2001). Furthermore, 3-month-olds learn about the visual appearance of a single or compound object striking a surface and the particular sound it produced when synchrony is present (Bahrick, 1988, 1992, 1994). Even newborns show evidence of learning arbitrary audiovisual relations in the presence of amodal information, but not in its absence (Slater, Quinn, Brown, & Hayes, 1999). Based on studies of a variety of events perceived through various sensory modalities, detection of amodal information such as temporal synchrony apparently can promote further differentiation of the unitary event and lead to learning about nested properties and finally about arbitrary, modality-specific relations. This developmental sequence whereby learning progresses in order of increasing specificity (from detection of amodal to arbitrary, modality-specific relations) is adaptive because it can promote learning about consistencies and regularities across the senses that are context independent prior to learning about more context dependent relations. Early detection of amodal relations fosters appropriate, veridical generalizations (e.g., voices go with faces; single objects produce single sounds) and minimizes inappropriate generalizations about relations that vary across contexts and are specific to certain events. In this manner, detection of amodal relations can guide and constrain learning about modality-specific relations such that general principles are well established prior to learning about more specific details of these events (see Bahrick, 2000, 2001). This sequence of increasing specificity and amodal processing precedence observed across development is likely paralleled by a similar processing sequence that occurs within a given episode of exploration. If an event provides intersensory redundancy and captures attention, then, according to the intersensory redundancy hypothesis, attention will first be focused on global amodal relations, followed

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by nested amodal relations, and eventually, on modality-specific properties. Once the infant has explored the redundant amodal relations uniting the multimodal stimulation from an event, this promotes continued processing of the unitary event and guides attention to increasingly more specific levels of stimulation. Thus, just as perception appears to proceed from detection of global to specific aspects of stimulation across development, perceptual processing within a given episode of exploration may also proceed from global to specific aspects of stimulation at a given point during development. Interestingly, a global processing precedence within an episode of exploration has been demonstrated for exploration of unimodal visual stimuli (see Freeseman, Colombo, & Coldren, 1993; Frick, Colombo, & Allen, 2000). Whether such a global processing precedence also holds for exploration of multimodal stimuli within an episode of exploration is an important empirical question and should be tested across a variety of domains. If the global-to-specific sequence previously described reflects a general pattern of multimodal processing, then the advantages of intersensory redundancy should be most evident and pronounced during early phases of exploration or processing of a particular event. Similarly, our intersensory redundancy hypothesis predicts that the facilitating effects of intersensory redundancy on differentiation of amodal stimulus properties should be most apparent early in development when infants are first learning a skill and when detection of the information is difficult. Once the skill is mastered and/or perceptual discrimination becomes easier, the information in question (for example, a particular rhythmic sequence) can be detected rapidly and will likely be discriminated in both unimodal and bimodal stimulation. Thus, the effects of intersensory redundancy should be most apparent when the organism is first learning to differentiate novel or relatively unfamiliar information. If the effects of redundancy are most pronounced in early development, this could have important implications for attention, perception, learning, and memory. Because these capabilities emerge primarily in a multimodal context, and initial conditions are known to have important influences on the trajectory and organization of subsequent development, the early effects of intersensory redundancy are likely to have a significant influence on the nature and course of later perceptual and cognitive development across a variety of areas. The intersensory redundancy hypothesis could thus potentially serve as a model to guide appropriate interventions for developmental delays in a number of perceptual, cognitive, and social domains as a function of the type of property in question (amodal vs modality-specific) and the likely basis of the particular developmental delay. B. DIRECTEMPIRICALSUPPORTFOR THE INTERSENSORY REDUNDANCYHYPOTHESIS The intersensory redundancy hypothesis allows several a priori predictions regarding how young infants should attend to and discriminate different patterns of

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multimodal and unimodal sensory stimulation. First, perceptual processing and learning about amodal properties of objects and events (e.g., tempo, rhythm, duration) will be facilitated when the properties are presented across two sense modalities in a temporally coordinated manner relative to when the same properties are available to only one sensory modality. Conversely, perceptual processing and learning about modality-specific properties (e.g., color, pitch, orientation) will be facilitated when the properties are presented to only one sense modality relative to when the same properties are presented to two or more senses concurrently (see Figure 2). Thus, the rhythm or tempo of a unimodal event should be less likely to be noticed and remembered than the rhythm or tempo of a bimodal event. The intersensory redundancy hypothesis also makes a developmental prediction. Because the facilitating effects of intersensory redundancy are likely most pronounced when infants are first learning a skill, we would expect differentiation of amodal properties to be extended more flexibly as infants become more competent perceivers. Specifically, we predict that in early development the detection of amodal properties should be facilitated by bimodal redundant stimulation when the information is novel or difficult, but detection of amodal properties will be extended to unimodal contexts in later development as infants become more skilled perceivers. Given the traditional dichotomy between research in the areas of unimodal and multimodal functioning, few studies in the existing literature relate directly to these predictions. Thus we have generated a body of research from our labs that tests these predictions of the intersensory redundancy hypothesis with both human and animal infants. 1. H u m a n - B a s e d Studies

Several studies with human infants provide support for specific predictions of the intersensory redundancy hypothesis. Bahrick and Lickliter (2000) assessed the ability of 5-month-old human infants to discriminate complex rhythmic patterns in bimodal, redundant stimulation as compared with unimodal stimulation. Infants were habituated to videos of a plastic hammer tapping out a distinctive rhythm under conditions of bimodal, redundant stimulation (they could see and hear the hammer), unimodal visual stimulation (they could only see the hammer tapping), or unimodal auditory stimulation (they could only hear the soundtrack of the hammer tapping). Infants were then presented test trials consisting of the same hammer tapping out a new rhythm. In this paradigm, following habituation (decreased interest in a familiar event), renewed interest (visual recovery) to a new event is taken as evidence of discrimination between the two events. Infants who received the bimodal, redundant stimulation showed significant visual recovery to the change in rhythm (discrimination), whereas those who received unimodal visual or unimodal auditory stimulation showed no visual recovery to the change in rhythm. In a further experiment we assessed infants' rhythm discrimination for bimodal, nonredundant stimulation (asynchronous films and soundtracks of

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the hammers tapping) and similarly found no evidence of rhythm discrimination. These results demonstrate that infants required redundancy in the form of temporal synchrony between the visual and acoustic stimulation to show a visual recovery to a change in rhythm (Bahrick & Lickliter, 2000). Thus, 5-month-olds discriminated complex amodal rhythms when they were bimodal and synchronous (seen and heard), but not when they were unimodal (seen or heard) or bimodal but asynchronous. This finding supports the first tenet of the intersensory redundancy hypothesis. In a second study we replicated and extended the findings of Bahrick and Lickliter (2000) by assessing detection of a different amodal property with infants of a younger age. Bahrick, Flom, and Lickliter (2002) tested discrimination of tempo in 3-month-old infants, utilizing the same basic paradigm described earlier. Infants were habituated to films of a hammer tapping out a rhythmic sequence in one of two tempos (55 vs 120 beats per minute). The same tempo could be detected visually by watching the hammer, or acoustically, by listening to its impact sounds. Infants received bimodal, redundant, audiovisual stimulation, unimodal visual stimulation, or unimodal auditory stimulation during habituation. Then they received test trials presenting a novel tempo. Results paralleled those of Bahrick and Lickliter (2000), in that infants discriminated the change in tempo following bimodal, redundant audiovisual stimulation, but not following unimodal visual or unimodal auditory stimulation. These findings converge with those of rhythm discrimination and demonstrate the facilitating effects of intersensory redundancy for guiding attentional selectivity and fostering perceptual differentiation in infancy. However, it is not clear to what extent the facilitating effects of intersensory redundancy persist across later stages of development. According to the intersensory redundancy hypothesis, the advantage of intersensory redundancy should be most pronounced when infants are first learning a skill and attenuated in later development. Once infants become proficient at detecting a particular stimulus property, perception of that property should become increasingly flexible and should no longer require redundancy. An amodal property may then be detected in unimodal stimulation. Bahrick, Lickliter, and Flom (2002) tested this hypothesis. We assessed the ability of 8-month-old infants to discriminate complex rhythms in a task just like that experienced by the 5-month-olds in our prior study. As before, infants received videos of the hammer tapping out one of the two rhythms. Infants were habituated to the rhythmic sequences in bimodal, redundant audiovisual stimulation or unimodal visual stimulation. Test trials presented a novel rhythm. The 8-month-olds showed significant visual recovery to the change in rhythm in both the redundant audiovisual and the unimodal visual conditions. These findings contrast with those of the 5-month-olds in the Bahrick and Lickliter (2000) study and demonstrate that by 8 months of age, infants no longer required intersensory redundancy for discriminating the complex rhythmic sequences. This finding provides support for

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our developmental prediction that perception of amodal properties emerges in the context of redundancy and is later extended to nonredundant, unimodal contexts. A few studies from related content areas have also provided unimodal and multimodal conditions to young infants and therefore provide converging evidence regarding the first prediction of the intersensory redundancy hypothesis--that discrimination of amodal information should be facilitated in bimodal as compared to unimodal presentations when performance is not at ceiling. Caron, Caron, and MacLean (1988) found that 7-month-old infants could discriminate happy from angry expressions spoken across different individuals when the face and voice were presented redundantly and synchronously, but not when the moving face was presented without the voice. Furthermore, discrimination of emotional expressions was significantly better in the bimodal compared with the unimodal condition, providing additional support for the salience of intersensory redundancy in a paradigm using social stimuli. Relatedly, Walker-Andrews (1997) comprehensively reviewed the literature on infants' affective discrimination and concluded that recognition of affective expressions emerges first in a multimodal context, and subsequently occurs on the basis of vocal and then facial expressions later in development. This developmental trajectory parallels the findings from our laboratory utilizing nonsocial stimuli. Additional research has also focused on a second tenet of the intersensory redundancy hypothesis, namely the perception of modality-specific properties of events perceived in unimodal versus bimodal, redundant stimulation (the fighthand quadrants of Figure 2). According to the intersensory redundancy hypothesis, information experienced in one sense modality selectively recruits attention to modality-specific properties of events and facilitates perceptual differentiation of those properties at the expense of other properties. To evaluate this hypothesis, we again tested 5-month-old infants, this time assessing detection of orientation, a property available visually, but not acoustically (Bahrick, Lickliter, & Flom, 2000). We expected that infants would discriminate changes in orientation during unimodal visual, but not bimodal audiovisual stimulation. Infants were habituated to films of a hammer tapping out a rhythm. However, this time the movements of the hammer were depicted in one of two orientations (upward vs downward). Either the hammer hit downward against a wooden floor, or it hit upward against a wooden ceiling. Infants were habituated to videos of the hammers in one of the two orientations in the bimodal, audiovisual condition (where they could see and hear the hammer moving) or the unimodal visual condition (where they could only see the hammer moving). Infants then received test trials, under their respective conditions, where the orientation of the hammer was changed. Infants detected the change in orientation (i.e., significant increase in looking) following unimodal visual habituation, but not following bimodal audiovisual habituation (Bahrick et al., 2000). Thus, consistent with predictions of the intersensory redundancy hypothesis, 5-month-olds successfully discriminated changes in orientation, a visual

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property, following unimodal visual exposure, but not following redundant, bimodal exposure. The addition of the soundtrack apparently created intersensory redundancy and selectively recruited attention away from unimodally conveyed properties and toward redundantly specified properties of stimulation (as in Bahrick & Lickliter, 2000; Bahrick, Flom, & Lickliter, 2002). In contrast, the unimodal, visual stimulation promoted attention to visual properties of the event without competition from salient redundant properties. Thus, attention to modality-specific or nonredundantly specified properties is likely best fostered in the context of unimodal exploration when competition from concurrent redundantly specified properties is minimized. This suggests that unimodal exploration (of a face or voice, for example) is promoted when there is little competition from concurrent, amodal, redundant stimulation (face-voice synchrony, for example). Unimodal exploration likely fosters differentiation of increasingly more specific aspects of auditory or visual information, a process that is not initially promoted when redundant stimulation is available. Thus, differentiation of the appearance of a face would be best promoted when the face is silent and relatively still, whereas when the individual is speaking and moving, competition from audiovisual redundancy would be more likely to focus attention on amodal properties such as prosody, rhythm, tempo, affect, and intensity variations common to the speech and facial movement (Walker-Andrews, 1997; Walker-Andrews & Bahrick, 2001). 2. Animal-Based Studies

If the intersensory redundancy hypothesis relfects a general developmental principle, then redundancy should potentially be a potent contributor to perceptual responsiveness and learning at earlier stages of development and in other animal species. Studies of nonhuman animal infants and human infants have shown sensitivity to amodal stimulus properties in the days and weeks following birth (Bahrick, 1988; Lewkowicz, 2000; Mellon, Kraemer, & Spear, 1991; Slater et al., 1999; Spear & McKinzie, 1994), but little is known about whether embryos or fetuses are sensitive to redundantly specified information during the prenatal period. Systematic manipulation of the human fetus's sensory experience during the prenatal period is, of course, prohibited and the use of such experimental methods is possible only with nonhuman animals. Lickliter, Bahrick, and Honeycutt (2002a) assessed whether redundant, bimodally specified information can guide attentional selectivity and facilitate perceptual learning prior to hatching in a precocial avian species. Precocial birds (such as domestic chicks, ducks, and quail) are particularly well suited for this type of research, as they develop in an egg (allowing ready access to the developing embryo during the late prenatal period) and can respond in behavioral tests immediately after hatching. In the Lickliter et aL (2002a) study, bobwhite quail chick embryos were exposed to an individual maternal call for 6, 12, or 24 h, under conditions of unimodal

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auditory stimulation, concurrent but asynchronous auditory and visual stimulation, or redundant and synchronous auditory and visual stimulation. Redundant stimulation was provided by presenting a pulsing light that flashed in synchrony and with the same temporal patterning (rhythm, rate, and duration) as the notes of the maternal call. Quail embryos are able to perceive the call and the patterned light during the late stages of incubation, after they have moved their head into the airspace at the top of the egg in preparation for hatching. All chicks were then tested 24 h later (1 day after hatching) to determine whether they would prefer the familiar bobwhite maternal call over an unfamiliar variant of the maternal call. Chicks that received redundant audiovisual exposure preferred the familiar maternal call following all prenatal exposure durations, whereas chicks that received nonredundant audiovisual exposure prenatally showed no preference for the familiar call after any exposure duration. Chicks receiving the unimodal auditory familiarization prior to hatching preferred the familiar call only following the longest period (24 h) of prenatal exposure. Thus, bobwhite quail chicks show dramatically enhanced learning of the maternal call when amodal information (tempo, rhythm, duration) is presented redundantly across two sense modalities. Embryos exposed to redundant presentation of auditory and visual information learned the maternal call four times faster than embryos exposed to unimodal auditory information. These findings are the first to demonstrate the facilitating effects of intersensory redundancy during the prenatal period and in a nonmammalian species. Similar to the results from human infants reviewed earlier, avian embryos showed enhanced perceptual learning when amodal information was presented bimodally and in a temporally coordinated manner. In a related study we also demonstrated that quail embryos provided intersensory redundancy prenatally show enhanced memory for the familiar maternal call in the period following hatching (Lickliter, Bahrick, & Honeycutt, 2002b). Specifically, chicks that received redundant audiovisual exposure as embryos were able to remember and prefer the familiar maternal call 4 days following hatching. In contrast, chicks receiving unimodal (auditory only) exposure prenatally failed to remember the familiar maternal call beyond 1 day following hatching. These results are the first both to provide evidence that redundantly specified information is remembered longer than the same information presented unimodally and to provide additional support for the facilitative effects of intersensory redundancy in the domains of perception, learning, and memory, even during the period prior to birth or hatching. Although little if any research has focused on this issue, the human fetus likely experiences redundancy across auditory, vestibular, and tactile stimulation in utero. For example, when the mother walks, the sounds of her footsteps can be coordinated with tactile feedback as the fetus experiences changing pressure corresponding with the temporal patterning and shifting intensity of her movements as well as the accompanying and coordinated vestibular changes. In addition, the mother's

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speech sounds, laughter, heartbeat, or sounds of breathing may create tactile stimulation that shares the temporal patterning of the sounds as a result of changes in the musculature involved in producing the sounds. Research has suggested that fetuses can discriminate auditory stimulation on the basis of temporal patterning such as prosody (DeCasper, Lecanuet, Busnel, Cranier-Deferre, & Maugeais, 1994; DeCasper & Spence, 1991). The infant may also experience self-produced intersensory redundancy between proprioceptive and tactile stimulation. Fetuses are known to engage in spontaneous motor activity of limbs and body (Robertson & Bacher, 1995), providing temporally organized cyclic stimulation. When the fetus moves in the uterus, the movement generates both proprioceptive feedback as well as temporally coordinated tactile consequences of the motion, such as changes in pressure on the skin. Additionally, the mother also responds with temporally coordinated movements to externally generated sounds. For example, she may dance or exercise to music, startle to a loud noise, or engage in conversation that has a distinctive turn-taking contingent structure--all of which produce movements that have tactile and/or vestibular correlates that share intensity and temporal patterning with the sounds. Thus, the developing fetus likely has ample opportunity to become familiar with and detect redundant stimulation during the late stages of prenatal development. Taken together, converging evidence across species, developmental periods, and properties of events indicates the importance of intersensory redundancy for promoting attention and for fostering perceptual differentiation of amodal properties of events. Further, intersensory redundancy explains how, in a predominately multimodal environment, perceptual learning can initially be guided and constrained by detection of amodal relations. These findings also reveal conditions under which attention to arnodal properties is not facilitated and attention to modality-specific properties and nonredundant aspects of stimulation are favored. That is, when a given event provides stimulation to only a single sense modality, attention and learning about unimodally specified properties of events is more enhanced. Modality-specific properties are likely best differentiated when competition from intersensory redundancy is not present. These findings highlight that in early infancy, perceptual development is characterized by a dynamic interaction between attention to amodal and modality-specific properties available in unimodal and multimodal stimulation. C. ON WHAT BASIS DOES INTERSENSORY REDUNDANCY FACILITATE PERCEPTUAL DISCRIMINATION AND LEARNING?

Although the salience of intersensory redundancy for perception, learning, and memory is now documented in studies of both neural and behavioral responsiveness, the basis for its salience and facilitation of perceptual processing is not yet clear. Theorists have proposed several explanations of the perceptual facilitation

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found for multimodal stimuli over unimodal stimuli. One way of conceptualizing the nature and salience of intersensory redundancy is illustrated in "separate activation" models (e.g., Estes, 1972, 1974; Rumelhart, 1970; Shaw, 1978; Shiffrin & Schneider, 1977). This type of model has been used in information-processing studies of adults' attention to address why processing of redundant signals in separate channels is faster than processing single signals (the "redundant signals effect"; Kinchla, 1974). According to this model, when redundant information is available in two separate channels, activation does not combine across the channels to allow for faster processing. Rather, processing of the information is faster when redundancy is present because there are two different opportunities to detect the signal, one in each channel. The faster of the two signals is assumed to win out for attention and to influence the response (also called the "race model"; see Raab, 1962). Across multiple trials, the time of the "winner" will be less than the average time of either signal. In the processing of multimodal redundant stimulation, such as rhythm or rate, presumably intersensory redundancy would be superior to unimodal processing because the perceiver could respond by detecting the information in either the auditory or visual channel. Thus, there would be a better chance of detecting the rhythm when it was redundantly specified. Available neural evidence and behavioral data on infant perception of intersensory redundancy do not provide support for the separate activation model and thus it is not the explanation we favor. As described in earlier sections, there appears to be considerable interaction among the sensory channels, even in early development, and information in one sensory modality significantly influences how information in other modalities is perceived, learned, and remembered. Another way of conceptualizing the salience and facilitating effects of intersensory redundancy, also used in studies of divided attention, is seen in "coactivation models," where both components of a redundant signal are seen to influence responding together (e.g., Logan, 1980; Miller, 1982; Nickerson, 1973). According to this perspective, both stimulus components combine to influence activation. Activation builds gradually over time and when it reaches a threshold, a response (such as word recognition) occurs. From this view, responses to redundantly presented signals are especially fast because both signals independently produce activation. When applied to intersensory perception, redundant information would be thought to be particularly salient and to enhance learning because there would be more activation and stimulation from two sense modalities than from either sense modality alone. Thus, the difference between multimodal and unimodal stimulation is seen as one of quantity and is based on overall amount of stimulation. The problem with this account is that the facilitation due to intersensory redundancy cannot be reduced to a simple quantitative benefit. Rather, as we reviewed earlier, there are multiplicative or superadditive effects in intersensory facilitation beyond what would be expected from adding together the effects of each modality alone (Stein & Meredith, 1993). Furthermore, how stimulation in the

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two senses is presented is also crucial for facilitation resulting from intersensory redundancy. For example, audible and visible stimulation must be temporally aligned (synchronous) for intersensory redundancy to be effective in guiding selective attention and perceptual responsiveness in infancy (Bahrick & Lickliter, 2000; Lickliter, Bahrick, & Honeycutt, 2002a,b). Thus, synchronous (but not asynchronous) presentations of auditory and visual stimulation results in the discrimination of changes in rhythm or tempo, even though the overall amount of stimulation is constant across the two conditions. Thus, the coactivation view, though consistent with some of the data from neural and behavioral studies, cannot account for reported qualitative differences between unimodal and multimodal stimulation. A third view of the role of intersensory redundancy in facilitating attention and perception, which can be described as "intersensory convergence," fits best with existing evidence from neural, physiological, and behavioral studies and is consistent with E. J. Gibson's (1969) invariant detection view of perceptual development. According to this perspective, perceivers are sensitive to higher order, amodal patterns of stimulation across the senses. When attention is focused on this type of redundancy, the perceiver detects a whole that differs from the sum of its parts. Pick up of concurrent, redundantly specified information causes amodal properties of stimulation to become "foreground" and other properties to become "background." The resulting information is not only quantitatively different from that conveyed in each sense modality alone, but qualitatively different as well. A metaphor from the area of visual perception may be useful in explaining this qualitative difference. The figure-ground segregation, or "pop out" effect for amodal properties of objects and events that results from detecting intersensory redundancy can be viewed in a manner similar to binocular convergence and resuiting stereopsis (e.g., J. J. Gibson, 1950; Sekuler & Blake, 1990). The two eyes see objects and events from slightly different angles because of their different positions in the head. Consequently different patterns of stimulation reach each of our retinas. When two eyes converge properly on a target, we see depth from binocular disparity. Furthermore, the impression of depth differs qualitatively from each of the individual component patterns. Perceivers have no awareness of either individual pattern alone. Rather, from the interaction between the two patterns of stimulation, perception of three-dimensional space emerges. For example, when viewing a three-dimensional object of a particular shape, the component patterns to each retina differ in form, but contain information for the same object shape. To perceive the emergent three-dimensional shape, the two patterns must be spatially aligned. That is, binocular convergence allows the two patterns to overlap properly, creating the perception of depth. We suggest that binocular convergence and stereopsis resemble the "pop-out" effect we experience for amodal stimulus properties when auditory and visual stimulation is redundant, concurrent, and temporally aligned. In the perception of multimodal events, the component patterns of stimulation to different sense

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modalities depict the same stimulus properties (e.g., rhythm, rate), but differ in their form (i.e., modality). To perceive the amodal properties of objects and events, the unimodal components must be temporally aligned. In other words, the convergence of different types of energy makes amodal properties of stimulation salient and fundamental for perception. We favor this convergence perspective because it is most congruent with evidence of intersensory interactions across multiple levels of analysis. First, it is most consistent with evidence from the neurosciences regarding sensory interactions. For example, the findings reviewed earlier regarding the "superadditive" effects of concurrent bimodal stimulation on neural responsiveness (e.g., Stein & Meredith, 1993) point to the fact that the convergence of different types of sensory stimulation can result in outcomes that are greater and different from the sum of its parts (that is, responsiveness levels are seen that would not be predicted from adding together input to either sensory modality alone). Second, the convergence view is consistent with psychobiological studies demonstrating the functional distinction between unimodal and multimodal stimulation (e.g., Lickliter & Honeycutt, 2001; Reynolds & Lickliter, 2002; Sleigh, Lickliter, & Columbus, 1998). This body of work has shown the importance of type and timing (qualitative aspects) of sensory stimulation as well as amount (quantitative aspects) on developmental outcomes. Third, the convergence view provides a framework that easily accounts for auditory and visual illusions (e.g., McGurk effect, bounce illusion). Finally, the convergence view is most consistent with behavioral data from young infants described in earlier sections, including all direct evidence reviewed in support of the intersensory redundancy hypothesis. It also addresses why synchronous audiovisual stimulation is far better in promoting differentiation of amodal properties than are nonsynchronous pairings of the same audiovisual stimulation. The convergence view thus accounts for both qualitative and quantitative differences between effects of unimodal versus multimodal sensory stimulation.

VI. Summary and Directions for Future Study of Perceptual Development Research on perceptual development reviewed in this chapter leads to several generalizations. These include: 9 The brain is inherently multimodal in both structure and function 9 There are superadditive effects in neural responsiveness to multimodal stimulation 9 There are strong intersensory connections and interactions; auditory information influences how visual information is perceived, and vice versa (this allows for various intersensory illusions)

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9 The salience and facilitative effects of intersensory redundancy is seen across species, from avian to mammalian (including humans) 9 Perception of amodal stimulus properties is promoted by concurrent, bimodally specified presentation of the same information in two or more sensory modalities (relative to unimodal presentations of the same information) 9 Perception of modality-specific stimulus properties is promoted by unimodal presentations (relative to bimodal presentation of the same information) Given the dramatic interconnections among the senses at all levels of analysis, from single-cell recordings to responses of neural populations, from attention to perceptual differentiation, learning, and memory, developmental psychologists can no longer ignore the importance of intersensory influences on basic processes of attention, perception, and cognition. Any account of development that aspires to be ecologically relevant and biologically plausible must be consistent with data from both the neural and the behavioral sciences regarding the basic role of multimodal stimulation in guiding and constraining individual development. We have reached a point in the study of infancy where "what" questions are being replaced by "how" questions (Lewkowicz, 2000; Lickliter & Bahrick, 2000; Thelen & Smith, 1994). This shift in emphasis from descriptive to explanatory research requires convergence across levels of analysis, species, and methods. Given the explosion of data from the biological and behavioral sciences regarding the nature of intersensory functioning, increasing cooperation and coordination across disciplines will be needed to provide a unified theory of perceptual development. In this light, we conclude this chapter with several integrative themes that could contribute to the future study of perceptual development. First, research on unimodal and multimodal perception needs to be better integrated. The current dichotomy between these approaches impedes progress toward a unified theory of perceptual development. Better integration could be achieved by incorporating unimodal and bimodal conditions into single studies where uniform methods allow for meaningful comparisons and avoid generalizing research findings beyond the context (unimodal vs multimodal) of investigation. Second, research from the neural and behavioral sciences needs to be better integrated. Developmental psychology can no longer ignore findings from neural and physiological levels of analysis showing the interrelation of the senses at primary levels of processing and the implications of this insight for behavior. Specifically, our appreciation of the multimodal nature of the brain points out that higher order perceptual processing is not needed to achieve integration (binding) across the senses. Given that the integration issue has guided theory construction in the study of perceptual development for the better part of the 20th century, new frameworks are needed that move beyond these old ways of thinking and successfully incorporate findings from the biological sciences. Similarly, the neural sciences can benefit from cross-fertilization with the behavioral sciences. Data generated

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from the behavioral level regarding the nature of intersensory functioning can and should inform neural studies. For example, direct investigations for the basis of the observed behavioral effects described by the intersensory redundancy hypothesis (the role of redundancy and synchrony in guiding selective attention and facilitating perceptual learning) are needed at both the neural and the physiological levels of analysis. Third, better integration of animal and human research is needed. The convergence of findings across different species will allow investigators to distill more fundamental developmental principles by highlighting invariant patterns of responsiveness that exist across species. Further, because we can experimentally manipulate the sensory experience of animals and thereby unpack the mechanisms of developmental change, the comparative approach can point to potentially fertile areas of investigation within the restrictions of human-based research (see Lickliter & Bahrick, 2000, for further discussion). Fourth, the role of prenatal development in shaping and guiding young infants' attention and perceptual processing can no longer be overlooked. Birth is not an adequate starting point for explanations of perceptual development (Lickliter, 2000). The infant has already had a great deal of prenatal sensory experience at the time of birth, and the nature and type of this prenatal experience must be taken into account when addressing the origins of intersensory functioning. As a case in point, newborns' demonstrated sensitivity to amodal information (e.g., Slater et al., 1999) likely has its roots in the detection of amodal stimulation in the prenatal environment. Fifth, the important role of selective attention could be better emphasized in developmental research concerned with perception and cognition. All information for perception and cognition must pass through the lens of selective attention. The natural environment provides an array of dimensions of stimulation, including unimodal-multimodal, moving-static, social-nonsocial, affectively laden-affectively neutral, and self-nonself. Which aspects or poles of these dimensions will be perceived, processed, and learned at different points in development is determined in large part by selective attention. Research is needed to define the salience hierarchies and rules that govern infants' deployment of attention in the natural flux of sensory stimulation that typically varies along these important experiential dimensions. The intersensory redundancy hypothesis provides one testable example of such a hierarchy. Sixth, further investigations of the interplay between the processing of modalityspecific and amodal aspects of stimulation are needed. The world of natural events can be described as providing modality specific and arnodal stimulus properties. When and under what conditions do infants attend to, perceive, or ignore each type of property and how does this affect learning and memory? The intersensory redundancy hypothesis described in this chapter provides one framework for guiding this type of research.

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ACKNOWLEDGMENT The writing of this chapter was supported by National Institute of Mental Health Grants MH26665 and MH26666.

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CHILDREN'S

EMOTION-RELATED

REGULATION

Nancy Eisenberg DEPARTMENT OF PSYCHOLOGY ARIZONA STATE UNIVERSITY TEMPE, ARIZONA 85287

Amanda Sheffield Morris DEPARTMENT OF PSYCHOLOGY UNIVERSITY OF NEW ORLEANS NEW ORLEANS, LOUISIANA 70148

I. DEFINITION OF EMOTION-RELATED REGULATION II. A BRIEF REVIEW OF VIEWS OF EMOTION REGULATION IN THEORIES OF EMOTION III. CONCEPTUAL ISSUES A. SITE OF REGULATION B. ANTECEDENT VERSUS CONCURRENT EMOTION REGULATION C. CONTROL VERSUS REGULATION D. VOLUNTARY OR EFFORTFUL CONTROL/REGULATION VERSUS LESS VOLUNTARY, REACTIVE CONTROL IV. AGE-RELATED TRENDS IN EMOTION-RELEVANT REGULATION A. RELIANCE ON EXTRAORGANISMIC VERSUS INTRAORGANISMIC REGULATION B. THE USE OF INTERNAL MENTAL OR COGNITIVE COPING STRATEGIES C. CHILDREN'S ABILITIES TO SELECT APPROPRIATE STRATEGIES FOR DIFFERENT STRESSORS V. MEASUREMENT OF EMOTION REGULATION A. SELF-REPORT MEASURES B. PARENT- AND TEACHER-REPORT MEASURES C. OBSERVATIONAL MEASURES D. PHYSIOLOGICAL MEASURES E. MULTIMETHOD APPROACHES VI. RELATION OF EMOTION-RELEVANT REGULATION TO QUALITY OF SOCIAL FUNCTIONING A. THEORETICAL EXPECTATIONS B. EMPIRICAL DATA 189 ADVANCESIN CHILDDEVELOPMENT AND BEHAVIOR,VOL.30

Copyright2002, ElsevierScience (USA). All rightsreserved. 0065-2407/02$35.00

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VII. RELATIONS OF DISPOSITIONAL RESILIENCY TO EFFORTFUL AND REACTIVE CONTROL AND SOCIOEMOTIONAL FUNCTIONING A. THE RELATION OF RESILIENCY TO REGULATION/CONTROL B. RESILIENCY AS A MEDIATOR OF THE RELATION OF EFFORTFUL ATYENTIONAL CONTROL TO SOCIAL COMPETENCE VIII. SUMMARY AND FUTURE DIRECTIONS REFERENCES

In the last 20 years of the 20th century, the role of emotion in psychology moved from a nuisance variable to a central construct; thus the literature on emotion is enormous and growing. Correspondingly, much theory and research addresses the regulation of emotion and of emotion-related behavior. In fact, Judy Dunn, in 1994, asserted that the topic of emotional regulation has received more attention than any other aspect of emotional development. Dunn's point is truer today (in 2002) than in 1994. In this chapter, we provide an overview of some of the issues and findings in developmental work on emotion regulation. We begin with a definition of emotionrelated regulation and provide a brief overview of some theories about emotion that are relevant to conceptions of emotion regulation. Emerging conceptual issues in the field are then discussed; next we consider age trends in emotion-related regulation, followed by a discussion of methodological and measurement issues related to the study of emotion regulation. We close with a discussion of the relation of emotion-related regulation to children's socioemotional functioning and suggestions for future research.

I. Definition of Emotion-Related Regulation Given the range of perspectives on the nature of emotion more generally (e.g., Ekman & Davidson, 1994; see later), we should expect that definitions of emotion regulation vary. However, they tend to contain some common elements-numerous capacities and processes are involved and emotion-related regulation is usually seen as goal-directed and functional in nature. For example, Thompson (1994) defined emotion regulation as the "extrinsic and intrinsic processes responsible for monitoring, evaluating, and modifying emotional reactions, especially their intensive and temporal features, to achieve one's goals" (pp. 27-28). He discussed various domains for emotion regulation, including neurophysiological responses, attention processes, construals of emotionally arousing events, encoding of internal emotion cues, access to coping resources, regulating the demands of familiar settings, and selecting adaptive response alternatives. Cicchetti, Ganiban, and Barnett (1991) defined emotional regulation as "the intra- and extraorganismic

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factors by which emotional arousal is redirected, controlled, modulated, and modified to enable an individual to function adaptively in emotionally arousing situations" (p. 15). They emphasized emotion regulation's role in maintaining internal arousal "within a manageable, performance-optimizing range" (p. 15). Building on the work of Thompson (1994), Cole and colleagues (Cole, Michel, & Teti, 1994), Campos Mumme, Kermoian, and Campos (1994), and others, we define emotion-related regulation as the process of initiating, avoiding, inhibiting, maintaining, or modulating the occurrence, form, intensity, or duration of internal feeling states, emotion-related physiological processes, emotion-related goals, and/or behavioral concomitants of emotion, generally in the service of accomplishing one's goals. This definition is so broad because, as we shall discuss, emotion-related regulation can occur in regard to numerous aspects of functioning and at various times in relation to the occurrence of emotion.

II. A Brief Review of Views of Emotion Regulation in Theories of Emotion A complete review of the theories of emotionality and emotional development is beyond the scope of this chapter. Nonetheless, a brief synopsis of some of the important theories of emotional development and the role of emotion regulation in these theories is of interest (for a more in-depth review, see Izard, 1977; Malatesta, Culver, Tesman, & Shephard, 1989). Most early theories of emotion were nondevelopmental (Malatesta et al., 1989). For example, the James-Lange theory of emotion asserts that the experience of an emotion is merely the awareness of physiological changes in response to arousing stimuli. In contrast, the attribution theory of emotion, postulated by Schachter and Singer, states that the physiological activation associated with various and even opposite emotions is similar in nature and that it is the cognitive attribution or evaluation concerning the causes of physiological response that determines which emotion is experienced (Bothamley, 1993). These debates about the nature of emotion are interesting in and of themselves, but they say little about the development of emotional experience or emotion regulation. Current theories of emotional development acknowledge that physiological, cognitive, social, and behavioral processes are all involved in emotion and emotionrelated regulation. According to Malatesta et al. (1989), three theories of emotional development are especially important: discrete emotions theory, cognitive/ constructivist theory, and attachment theory. We add to this triad a fourth theory~ the functional theory of emotions. These theories have implications for the ways in which the development of emotion regulation is understood and studied. Differences between these theories are primarily due to opposing views regarding the relation between the experience of emotion and behavior (feeling states vs facial and other expressive gestures) and the degree to which emotional behavior

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is guided by cognitive processes versus evolutionary, innate control systems in early development (Malatesta et al., 1989). Discrete emotions theory (Ekrnan & Davidson, 1994; Izard, 1977) reflects Darwin's beliefs about the origin of basic emotions and their importance in biological and social adaptation. One of the most influential theories of discrete emotions is Izard's (1977) differential emotions theory, which emphasizes a limited set of primary, discrete emotions, each with an individual, fundamental purpose. Discrete emotions theorists view emotions as adaptive signals that are present from birth. Each emotion has distinct facial, vocal, and gestural features that provide a common communicative link between individuals across cultures. Discrete emotions theorists propose that emotion regulation and emotional development are the products of the maturation of the nervous system, changing adaptational needs, and cognitive development. Despite the belief that the basis for emotionality is primarily biologically "hardwired," discrete emotions theorists state that socialization is important in emotional development and emotion regulation (Malatesta et al., 1989). Researchers with a theoretical bent toward discrete emotions theory most often examine emotion regulation via observations, focusing on physical indicators of emotionality and regulation such as facial expressions. Moreover, because emotions are believed to be innate, discrete emotions theorists sometimes study emotional reactions in early infancy. Cognitive/constructivist theorists of emotion emphasize the role of cognition in emotional development and emotion regulation and focus less on innate aspects of emotion. Sroufe's (1979) theory of emotional development is a good example of the cognitive/constructivist perspective. He suggested that emotions are initially undifferentiated states of distress and nondistress that later differentiate into specific emotions as a function of development. Differentiation of emotions and control over emotions occurs gradually, in a stagelike manner, as a product of cognitive development. Cognition acts as the central processor in the growth and elaboration of emotionality and emotion regulation (Malatesta et ai., 1989). This view of emotionality has important implications for studying the development of emotion regulation, suggesting that in early development the study of specific emotions is less important than examining overall distress. Furthermore, a cognitive/ constructivist researcher would be interested in the role of cognitive processes and maturation in emotion regulation as well as the subjective experience of an emotion and the processes individuals actively choose for managing their emotions. Bowlby's attachment theory, although not generally thought of as a theory of emotional development, has been used to derive hypotheses about the development of emotion regulation. Attachment theory combines psychoanalytic theory and ethology to explore the young child's emotional bond with caregivers and the role of these relationships in social and emotional development (for a complete review of Bowlby's theory, see Bretherton & Waters, 1985). Attachment theorists emphasize the adaptive nature of emotion because of its role in motivating behavioral processes and ensuring the safety of the infant.

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Furthermore, they highlight bioevolutionary processes and the role of emotion in the development of social relationships and personality. Attachment theorists argue that the development of emotion regulation is influenced primarily through the child's expectations ("working models") of caregiver behavior. Working models develop during the 1st year of life (although they may be modified somewhat thereafter), are based on repeated daily interactions with the caregiver, and provide the basis for future social and emotional expectations and interactions (Cassidy, 1994). A child who is securely attached develops an expectation that his or her emotional signals and needs will be reacted to by the caregiver in a responsive, consistent manner. Securely attached children learn to freely express their emotions, and the development of emotion regulation is enhanced because of the supportive emotional environment children experience (Calkins, 1994; Cassidy, 1994). In contrast, an insecurely attached child is believed to develop the expectation that emotional needs will not be responded to or will be responded to in a selective and inconsistent manner. When a child expects a caregiver to be unresponsive and inconsistent in this way, the child may restrict the display of emotions or display negative emotions to receive attention from the caregiver, and the development of emotion regulation suffers (Thompson, Flood, & Lundquist, 1995). Thus, attachment researchers who do examine emotion regulation (empirical research on the topic is limited) will likely focus on how parent--child relationships, including children's attachment status/behaviors or maternal sensitivity, relate to children's regulation and negative emotionality (Carter, Little, Briggs-Cowan, & Kogan, 1999; Conteras Kerns, Weirner, Gentzler, & Tomich, 2000; Diener & Mangelsdorf, 1999). Many researchers studying emotion today have adopted a functional view of emotion, viewing emotion as an integral part of personality, motivation, and functioning (Campos, Carnpos, Barrett, 1989). This view of emotion differs somewhat from earlier conceptualizations of emotion in which it was argued that emotions were personal subjective states, epiphenomenal, and "background noise" that muddied the study of behavior. Earlier views of emotion as a subjective, primarily cognitive state made the study of emotionality a very difficult task for researchers. Saarni, Mumme, and Campos (1998) argued that past definitions of emotion caused the study of emotional development to be stagnant for more than 40 years and that increased interest and research on emotions is due to the functional perspective on emotion. The functional view of emotion focuses on person-event transactions, the impact of emotion on behavior, and the relation between emotion and action or action tendencies. This perspective also stresses the important role of context and the relational nature of emotion. Thus, within the functional perspective, no longer is emotion purely intrapsychic. Emotion is closely tied to an individual's goals and interactions with others, and emotions are social signals that communicate information (Campos et al., 1989; Malatesta, 1990). Functional theorists view emotions as adaptive because they act as signals to the self and to others. For example, when an individual experiences fear, it signifies

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personal threat and promotes fight-or-flight responding. As a signal to others, fear indicates submission and usually has the adaptive consequences of warding off attack (see Saarni et al., 1998, for a list of signal properties of emotions and their adaptive functions). Nevertheless, emotions such as fear can be maladaptive in certain situations. For this reason, and because emotions create a setting for person-environment transactions, situations and transactions often require changes in the manifestation of the original emotion. In other words, the outcome of an emotion can be very different from its intended adaptive function, and emotionrelated regulation allows for control over the outcome of the expression and the experience of emotions (Campos et al., 1994). Thus emotion-related regulation occurs at various times in relation to emotional experience and can vary in its manifestations. The functional theory of emotion is not a developmental theory per se, but it does have important implications for understanding, interpreting, and measuring emotionality and regulation. Emotion regulation is seen as functional in most cases as long as the goal of the regulator is to manage emotions in an adaptive manner, in a particular context (Campos et al., 1989). The ways in which an emotion is regulated and the goals of the regulator depend somewhat on contextual variables, however. For example, a mother may yell frantically at her child because the child is about to step into a busy street. The mother's emotional response and goals in this situation are adaptive, even though in other situations such an emotional response may be less adaptive. In this sense, the functional perspective is important to consider in development because a prime objective in the development of emotion regulation is to learn adaptive, socially appropriate goals and ways in which to manage emotions in socially and contextually appropriate ways (Kopp, 1992). When examining emotion regulation from a functional perspective, researchers are likely to consider the goals and motives of the regulator as well as the specific context in which regulation is taking place. Furthermore, assessments that focus on the subjective experience of emotion or on facial expressions of emotions are less likely to be used by pure functionalists because these types of measures are not easily interpreted from a functional perspective.

lII. Conceptual Issues We have found a variety of distinctions to be helpful when thinking about the nature of emotion-related regulation. Several of these are now discussed briefly. A. SITEOF REGULATION Campos and colleagues suggested that emotion regulation can take place at three general loci: at the level of sensory receptors (input regulation), at central levels

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where information is processed and manipulated (central regulation), and at the level of response selection (labeled output regulation; Campos et al., 1994). Consistent with their thinking, we differentiate between the regulation of internal states and processes and the regulation of overt behaviors associated with emotion. The regulation of internal states often has been discussed by temperament researchers, most notably Derryberry and Rothbart (1997, 1988). They have operationalized regulatory systems as involving attentional processes such as the abilities to shift and focus attention as needed (Derryberry & Rothbart, 1988; Windle & Lerner, 1986). For example, individuals can avert their attention from a frightening film to cut off arousal or focus on neutral objects or thoughts and therefore modulate the form, intensity, or duration of internal feeling states and physiological processes. Similarly, coping theorists have examined cognitive coping processes such as cognitive distraction--thinking of something else to alleviate emotional arousal or distressmand positive cognitive restructuring--reframing the situation to highlight positive aspects of it (Lazarus & Folkman, 1984; Sandier, Tein, & West, 1994). In addition, as discussed by motivation theorists such as Heckhausen (Heckhausen, 1997; Heckhausen & Schulz, 1994) and Skinner (1999), one can also modulate negative emotion by modifying one's goals or their relative valuation (e.g., by devaluing the importance of success at work and by emphasizing the value of family and close friends in old age). The regulation of the behavioral concomitants of emotions--labeled for convenience as emotion-related behavioral regulation--includes control or modulation of facial and gestural reactions and other overt behaviors that stem from, or are associated with, internal emotion-related psychological or physiological states and goals (Eisenberg, Fabes, Guthrie, & Reiser, 2000). We consider this type of regulation to be an aspect of the broader construct of emotion-related regulation (as defined previously). Behavioral regulation is tapped by some temperament researchers who assess inhibitory or activation control, that is, the abilities to voluntarily inhibit or activate behavior. In addition, clinical, developmental, and personality theorists frequently have discussed constructs such as self-regulation, constraint, or ego control, which at least partly involve the ability to modulate or inhibit the behavioral expression of impulses and feelings (e.g., Block & Block, 1980; Tellegen, 1985). Although the distinction between regulation of internal states and external expressions of them is somewhat artificial because these processes are certainly somewhat intertwined, it has proved useful when examining various aspects of regulation (Eisenberg, Guthrie, et al., 1997; Eisenberg, Fabes, et al., 2000). B. ANTECEDENTVERSUSCONCURRENTEMOTIONREGULATION At least two other distinctions are important. The first is when the regulation occurs. Emotion-related regulation can occur prior, during, or after the elicitation

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of emotion (or its likely elicitation, which might be precluded). Usually investigators have studied regulation of emotion while it is being elicited or after it has been elicited. Regulation prior to its elicitation has been called proactive coping or antecedent emotion regulation (Aspinwall & Taylor, 1997; Carstensen, 1991; Gross, 1999; Thompson, 1994). Proactive coping, defined as "efforts undertaken in advance of a potentially stressful event to prevent it or to modify its form before it occurs" (Aspinwall & Taylor, 1997, p. 417), is viewed as virtually always active and as not involving positive reappraisals or other methods of emotion soothing. As an example, a shy person may arrange to have 6 rather than 15 people to a social gathering because he or she does not want to experience social anxiety, or a person may seek information to make decisions that avoid stressful events. Antecedent emotion regulation involves managing emotional reactions before they occur by using not only proactive coping but also attentional and cognitive processes to choose the situations that are focused on and how they are interpreted (Gross, 1999). For example, before the party, the shy person may frame the situation as a casual party with only close friends who happen to be work colleagues rather than as a dinner with colleagues that has implications for interactions at work. Investigators who argue a functionalist perspective on emotion take yet another perspective--they treat emotion regulation as part of the emotion-eliciting situation. According to this perspective, an individual's response to a given stimulus is viewed as intrinsically affected by the possibility of the response that the individual can make to the situation. Emotion and emotion regulation are not viewed as a sequential process because even the individual's initial response is affected by affordances of that situation--what might happen in the situation and what responses (including regulatory responses) the individual can make in the given context based in part on prior experience. As an example of how preexisting perceptions influence initial emotional responding, Gunnar (1980) found that 12-month-olds who had control over the activation of an arousing, potentially fear-provoking toy, in comparison to those who did not, were less fearful and more likely to approach the toy when it was making potentially frightening noises and movements (a similar reaction was not found for younger infants). Thus perceptions of the ability to regulate the environment influenced infants' initial fear response to activation of the toy. An important point stemming from this perspective is that emotion cannot be fully differentiated from emotion-related regulation because each affects the occurrence and form of the other and they are bound together and embedded in the social context. c. CONTROLVERSUS REGULATION Another useful distinction is between regulation and control, with the latter defined as inhibition or restraint (regardless of the cause of the inhibition). Control increases in a linear manner, from low restraint or inhibition to high restraint or

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inhibition. Successful regulation likely involves optimal levels of control as well as other capacities such as the ability to initiate action as needed. As with a number of other investigators (e.g., Block & Block, 1980; Cole, Michel, & Teti, 1994), we believe that well-regulated individuals are not overly controlled or undercontrolled; they have the capacity to respond to the ongoing and varying demands of experience with a range of responses that are both socially acceptable and sufficiently flexible to allow for spontaneity as well as the inhibition of spontaneous reactions as required in the given context. Whereas regulation generally is viewed as adaptive, it can be differentiated from the level of a child's control, which may be either adaptive or maladaptive, depending on its flexibility and if it can be voluntarily managed. D. VOLUNTARYOR EFFORTFULCONTROL/REGULATIONVERSUS LESS VOLUNTARY,REACTIVECONTROL A key distinction in thinking about the constructs of regulation and control is between voluntary or effortful control and less voluntary or reactive control. Well-regulated children would be expected to be relatively high in the ability to voluntarily control their attention and behavior as needed to respond in an adaptive manner. The construct of voluntary control overlaps substantially with what Rothbart has labeled as effortful control: "the ability to inhibit a dominant response to perform a subdominant response" (Rothbart & Bates, 1998, p. 137). Voluntary control involves the "effort" or "will" to control reactivity, including emotion-related aspects of temperament (Ahadi & Rothbart, 1994). Effortful control is reflected in effortful attentional regulation--the ability to voluntarily focus or shift attention as needed in a given situationmfor example, the ability to focus on some things rather than others as required to control one's experience or expression of emotion. Effortful control also is reflected in inhibitory and activation control, or the abilities to effortfully inhibit or activate behavior as appropriate, even if the person does not really want to do so. An example of inhibitory control is a person inhibiting his or her expression of anger; an example of activation control is getting oneself to do a task that induces anxiety. In contrast to effortful types of regulation, some aspects of control, or the lack thereof, seem to be involuntary or so automatic that they cannot usually be controlled voluntarily. These include some types of impulsivity and approach behavior (e.g., based on interest or potential rewards) or, at the other extreme, very low impulsivity as in overcontrolled children who are timid, constrained, and lack flexibility in novel or stressful situations. Derryberry and Rothbart (1997) distinguished active (effortful) control from more passive or reactive types of control and suggested that behavioral inhibition (i.e., inhibition and wariness in novel or challenging situations; Kagan, Snidman, & Arcus, 1992) is a good example of reactive overcontrol. Nigg (2000) and Mezzacappa, Kindlon, Saul, and Earls (1998) made

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similar distinctions between executive control and what they called motivational control (including behavioral inhibition). Effortful and less voluntary (i.e., reactive) aspects of control likely have different neural bases. Effortful control is believed to involve executive functioning in the cortex (Mirsky, 1996)--likely involving the anterior cingulate gyrusmwhich appears to be directly related to awareness of one's planned behavior (Posner & DiGirolamo, 2000). Posner has argued that the anterior cingulate gyms is linked to our subjective feelings of voluntary control of our thoughts and feelings and comes into play when resolving conflict, correcting errors, and planning new actions (Posner & Rothbart, 1998). In contrast, Gray (Picketing & Gray, 1999) and others have argued that impulsive (uncontrolled) and overly inhibited behaviors are associated with subcortical systems (e.g., the amygdala for inhibition and mesolimbic dopamine pathways for approach; Cacioppo, Gardner, & Berntson, 1999; also see Picketing & Gray, 1999), such as Gray's (1975, 1987) Behavioral Inhibition System (BIS), which is activated in situations involving novelty and stimuli signaling punishment or frustrative nonreward, and the Behavioral Activation System (BAS), which involves sensitivity to cues of reward and cessation of punishment. Of course, given the complexity of the human brain, cortical and subcortical functions have many neural connections. For example, approach and withdrawal systems are linked to asymmetries in frontal cortical brain activation (Tomarken & Keener, 1998). Therefore, the more effortfully controlled regulatory processes sometimes at least partly modulate the expression of less voluntary, reactive responses (see Derryberry & Rothbart, 1997). To summarize, current thinking about emotion-related regulation is complex and involves distinctions between effortful control or regulation and more reactive control, as well as distinctions among various mechanisms or processes used for regulation and their timing of occurrence. In general, the empirical work on emotion-related regulation does not fully reflect the level of conceptual thinking about the construct--but the field is moving in that direction.

IV. Age-Related Trends in Emotion-Relevant Regulation Researchers have noted several general age trends in emotion-relevant regulation during childhood (see Brenner & Salovey, 1997; Kopp & Neufeld, 2002; Walden & Smith, 1997).

A. RELIANCEON EXTRAORGANISMICVERSUS INTRAORGANISMICREGULATION One age-related trend involves the transition from infants relying almost totally on other people to help them regulate their emotion to their increasing ability

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to self-regulate during early childhood--that is, to greater reliance on solitary or intraindividual strategies of regulation/control. In the first months of life, caregivers help infants regulate their emotional arousal by controlling their exposure to stimulating events (Gianino & Tronick, 1988; Sander, 1964). When an infant is distressed, frustrated, or frightened, parents also typically try to soothe or distract the baby (Fogel & Thelen, 1987). Moreover, caregivers' own expression of emotion can arouse or calm infants; for example, by 5 months, infants display different emotional reactions to adults' positive and negative vocal affect (Fernald, 1993). Changes in infants' physiology may contribute to parents' success at helping infants to regulate their emotion. Thompson (1994) argued that early in life, the excitatory processes that underlie arousal become less labile; thus, children's susceptibility to arousal declines. This development is due to changes in postnatal functioning of the hypothalamic-pituitary-adrenocortical system that controls reactions to stress and to the maturation of parasympathetic regulation as indexed by vagal tone. As a consequence of these changes, infants' arousal becomes more gradual and motivationally complex, and the declining lability of reactivity aids caregivers' efforts to manage infants' emotional arousal. By the end of the 1st year of life, infants generally use caregivers' social signals to guide their interpretation of, and reactions to, potentially evocative events and objects. In studies of social referencing, infants' reactions in a situation of uncertainty, as when exposed to a stranger or a novel toy, are affected by their mothers' facial and vocal reactions to the situation (particularly vocal reactions; see Saarni et aL, 1998). For example, children approach a novel toy or visual cliff more quickly and more often if their mothers display a happy rather than a fearful face and provide analogous vocal signals (see Campos, Barrett, Lamb, Goldsmith, & Stenberg, 1983; Saarni et al., 1998; Moses, Baldwin, Rosicky, & Tidball, 2001). Moreover, if mothers express negative emotion (e.g., disgust) in reaction to a toy, children are less likely to play with the toy, even after their mother has stopped expressing the negative emotion (Hornik, Risenhoover, & Gunnar, 1987). The social support provided by adults' presence also appears to help the young child to regulate his or her emotion. In a study by Gunnar, Larson, Hertsgaard, Harris, and Brodersen (1992), 9-month-olds who interacted with a responsive, warm substitute caregiver during a separation from mother exhibited less negative emotion and physiological (i.e., adrenocorticol) reactivity than did infants with a substitute caretaker who was warm when the infant was distressed but relatively unavailable when the infant was not distressed during the separation period. In the early years of life, infants and toddlers become more capable of actively (rather than passively) eliciting social assistance with regulation. By 6 months of age, children take a more active role in social exchanges involving emotion

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(e.g., smiling) and increasingly attend to their mother when she is expressing emotion (Kaye & Fogel, 1980). Moreover, communicative cries involving visual regard and gestures toward another become more frequent between 9 and 12 months (Gustafson & Green, 1991), suggesting that infants expect assistance from their caregivers when they are upset. Rothbart, Ziaie, and O'Boyle (1992) found that 10-month-olds were more likely than younger children to use social communication in potentially stressful situations. Moreover, in another study, when children were told to wait for an attractive object, efforts to engage others in play or conversation increased from 12 months to 3.5-4.5 years of age (Bridges & Grolnick, 1995). The ability to use other self-originating methods of regulation also increases in the early years of life (Mangelsdorf, Shapiro, and Marzolf, 1995). According to Kopp (1989), in the first 3 months of life, children's emotion regulation consists of simple behaviors and reflexes, and is completely unplanned. For example, infants regulate by turning their head away from a distracting stimulus or by sucking on their hand to self-soothe. Between 3 and 7-9 months, cognition begins to play a greater role in emotion regulation. Infants become aware of different arousal states and realize that their arousal can be influenced by the actions of others and themselves. Toward the end of the 1st year of life, emotion regulatory abilities expand dramatically. Infants increase their own repertoire of emotion regulation techniques by utilizing some of their newly acquired cognitive skills (e.g., the ability to anticipate events or actions). For example, at this age, in comparison to younger ages, infants are somewhat more capable of distraction and shifting attention away from distressing stimuli. Consistent with Kopp's assertions, infants' regulatory efforts seem to become more effective with age. For example, children's use of self-comforting (e.g., by clasping of hands, hair, face, feet or sucking of fingers or thumbs) seems to be associated with declines in emotional arousal for 10- but not 5-month-old infants (Stifter & Braungart, 1995). When the child is engaged during a frustrating delay task, older toddlers and preschoolers appear better able than younger children to distract themselves by playing on their own and are less likely than younger children to seek comfort from a preoccupied parent (Bridges & Grolnick, 1995). The use of such distracting behaviors rather than seeking comfort from parents tends to be associated with lower levels of negative emotionality in young children (Bridges, Grolnick, & Connell, 1997). Moreover, with age, toddlers and young children increasingly use language to negotiate with their environment rather than merely expressing their frustration or anger through venting of emotion (Kopp, 1992; Kopp & Neufeld, 2002). Children's abilities to use language regarding emotion increase between 18 and 24-30 months. During this time, children learn how to use emotion labels to comment on their own feelings and to communicate with others about emotion (Bretherton, Fritz, Zahn-Waxler, & Ridgeway, 1986). Such developments in language likely affect

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emotion-related regulation in at least two important ways: they allow children to share their feelings with others and to obtain verbal feedback and instruction regarding the regulation of emotion and associated behavior (Kopp, 1989). Thus, with age and increased regulatory capacities, young children appear to have less need to rely on adults to help them regulate their emotions. This trend continues into the school years. In elementary school, for example, younger children are more likely to report relying on adults for assistance when coping with stress or problems whereas older children report more solitary coping strategies (Bernzweig, Eisenberg, & Fabes, 1993; Kliewer, 1991; Ryan, 1989). Children's increasing ability to regulate themselves is based partly on changes in their effortful control as well as on developmental changes in representational skills, memory, and the cognitive understanding of means-end relationships and other aspects of the physical and social world (Kopp & Neufeld, 2002). Executive attention, which involves effortful attentional control, is believed to show some development in the early months of life (e.g., as seen in orienting) and at approximately 18 months of age; however, it is still quite immature at 24 months of age. The ability to use executive attention seems to increase dramatically in the 3rd year of life (Posner & Rothbart, 1998). In regard to inhibitory control (i.e., the ability to effortfully inhibit behavior), the use of gestures or verbalizations to others regarding objects (e.g., forbidden objects), for example, indicates that infants can inhibit nonessential behaviors to perform desired actions, which is perhaps evidence of very rudimentary inhibitory skills (Kopp & Neufeld, 2002). In the 2nd year of life, infants are better able to control their motor behavior, for example, to slow down their walking or to stop an activity when asked to do so (Kochanska, Murray, & Harlan, 2000). This ability to effortfully inhibit behavior on tasks such as "Simon says" appears to improve considerably at approximately 44 months of age (Posner & Rothbart, 1998) and is fairly good by 4 years of age (Reed, Pien, & Rothbart, 1984). Although effortful control may be fairly well developed by 4 or 5 years of age, regulation in general improves across childhood and into adulthood (Murphy, Eisenberg, Fabes, Shepard, & Guthrie, 1999; Williams, Ponesse, Schachar, Logan, & Tannock, 1999). The increase in children's self-regulation may be due not only to age-related increases in infants' control of their attention and movements but also to changes in adults' expectations of them. As children age, adults increasingly expect children to manage their own emotional arousal and behavior. Campos and colleagues found that once children are capable of crawling, for example, they are viewed as more responsible for their behavior and for complying with parental expectations (Campos, Kermoian, & Zumbahlen, 1992). Kopp (1982) has argued that at about 9 to 12 months of age, children start to show an awareness of socializers' social or task demands and begin to initiate, maintain, modulate, or cease physical acts, communication, and emotional signals accordingly. If socializers do not start to allow their children to deal with their emotions themselves, they may prevent

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their children from developing regulatory skills. Grolnick, Kurowski, McMenamy, Rivkin, and Bridges (1998) found that children in the 2nd and 3rd years of life with mothers who were relatively active in dealing with their children's distresses were more distressed when they had to deal with their own distress independently. B. THE USE OF INTERNALMENTAL OR COGNITIVE COPING STRATEGIES The second, albeit related, trend in children's emotion-relevant regulation is an increase in the use of internal mental or cognitive coping strategies with age. Older children are better able to use cognitive strategies such as mentally distracting themselves from negative or stressful events or trying to see things in a positive light (Altshuler, Genevro, Ruble, & Bornstein, 1995). Even in the 2nd year of life, toddlers who focus on a commodity (e.g., food or a toy) that they want but cannot have and do not divert themselves with activities or other objects are likely to become distressed (Bridges, Grolnick, & Connell, 1997). In delay-of-gratification studies, Mischel found that 5-year-olds were more aware than were preschool children that looking at stimuli other than the desired food would help them to delay gratification and obtain more treats. By sixth grade, children were aware that thinking about desirable treats in an abstract manner unrelated to their desirability helps in resisting temptation (Mischel & Mischel, 1983). To resist pretzels, for example, older children may think about how pretzels could be used to build a toy log cabin rather than about how good they taste. Although few investigators have explicitly examined the processes mediating the shift toward greater use of mental regulatory strategies, the shift to the use of more mentalistic modes of coping is likely based, at least in part, on the increase with age in children's understanding of mental states and the nature of emotion (Eisenberg, Murphy, & Shepard, 1997). In the toddler and preschool years, representational skills and recall memory improve, so children can better understand internal states and notice consistencies in causes and consequences of events and behaviors related to emotions. Moreover, children's development of self-awareness allows them to understand that they not only feel distress but can minimize it through their own initiative (Kopp, 1989). In addition, children's understanding of display rules and the strategic control of emotional expressions in social interactions increases with age during the preschool and school years (Cole, 1986; Saarni, 1979, 1989), as does their awareness of how to use mentalistic strategies to regulate emotion (Harris, Olthof, & Meerum Terwogt, 1981; Terwogt & Olthof, 1989). For example, 5-year-olds are less likely than 8-year-olds to understand that people may be able to make themselves feel happy just by thinking about something happy. Part of the difficulty is that, compared to 8-year-olds, 5-year-olds are relatively unaware of how the stream of conscious thoughts affects people's feelings (Flavell, Flavell, & Green, 2001).

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Children's understanding of emotionality and its regulation probably continues to develop in late childhood and adolescence. Thompson (1991) argued that an individual's personal theory of emotion (i.e., one's theory about how emotions function and are managed in oneself) develops primarily during adolescence in conjunction with identity development. Because adolescence is a time when self-awareness and self-understanding are becoming more defined, beliefs about emotions and regulation also become more solidified (Thompson, 1991). Gottman and Mettetal (1986) emphasized social rather than intrapsychic processes during adolescence and suggested that important transformations in emotion regulation occur in middle childhood and adolescence as a result of social interactions with peers. For example, in studies examining the thematic content of friends' conversations, adolescents often talked about emotional topics and how to deal with emotional situations, such as romantic interests and conflicts with parents. In several cases, adolescents asked specifically for advice about how to recover from a bad mood (Gottman & Mettetal, 1986; Parker & Gottman, 1989). Furthermore, during adolescence peer relations become increasingly important and dependence on parents weakens. Adolescents tend to look unfavorably on parents' suggestions for coping and emotion regulation, unlike younger children (Garber, Braafladt, & Weiss, 1995). C. CHILDREN'S ABILITIES TO SELECT APPROPRIATE STRATEGIES FOR DIFFERENT STRESSORS

The third (and, again, related) trend in emotion-relevant regulation is children's growing ability to match appropriate strategies for dealing with situations to the nature of stressors. Older children seem better able to select ways of dealing with their concerns that are situationally acceptable and constructive (Berg, 1989). As one example, with age, children are better able to distinguish between stressors that can be controlled (such as homework) and those that cannot be controlled (such as painful medical procedures). Older children are more aware than younger children that in situations they cannot control, managing their emotion is easier by adapting to the situation rather than trying to change it (e.g., Altshuler & Ruble, 1989; Altshuler et al., 1995; Hoffner, 1993; Rudolph, Dennig, & Weisz, 1995). Faced with having to undergo major surgery, for instance, older children may adapt by trying to think about the positive consequences of having the surgery, such as being in better health afterward, or by distracting themselves by thinking about or engaging in enjoyable activities. Younger children, in contrast, are more likely to insist that they do not need the operation. Thus, an increase in the understanding of the nature of stressors and the range of means of regulating oneself allows older children to select more constructive and effective means of managing their emotions. In summary, rapid changes in children's ability to regulate their own attention, emotion, and expression of emotion occur in the 1st decade of life, especially in

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the first 4 years of life. These newfound abilities contribute to children's growing abilities to manage stress and emotion and to act in socially appropriate ways. Of course, individual children differ in both their voluntary (effortful) regulation and the degree to which seeming control is due to temperamental factors such as behavior inhibition and shyness. These differences have important implications for children's emerging social competence and for their adjustment.

V. Measurement of Emotion Regulation As we noted previously, the empirical work on emotion-related regulation is not as advanced as conceptual thinking about the construct. Indeed, when one peruses journal articles claiming to study emotion regulation, one sees that measures of emotion regulation obviously come in many different forms, including, but not limited to, indices of temperament, coping, vagal tone, facial expressions, intensity of expressed emotions, emotional lability, anger, and even aggression. These various measures may or may not tap what we have defined as emotion-related regulation, and clarification is greatly needed in the field to integrate findings across studies. One reason measures of emotion regulation differ so much is that investigators are interested in and/or choose to analyze different components and correlates of emotion regulation. For example, temperament theorists tend to use questionnaire measures that assess temperamental components of emotion-relevant control (e.g., effortful control, impulsivity, reactivity) and allow for generalizations across contexts and situations in an attempt to tap underlying constitutional processes important in emotion regulation (Rothbart Ahadi, Hershey, & Fisher, 2001). Researchers interested in the role of cognition in emotion-related regulation are likely to assess either dispositional or situational cognitive processes (e.g., how individuals appraise specific emotion-eliciting situations and use cognitive distraction or cognitive restructuring to modify its significance; Heckhausen, 1997; Lazarus & Folkman, 1984; Sandier et al., 1994). Furthermore, individuals interested in attachment relationships often observe the ways in which young children deal with different stressors or emotions when the parent is nearby (e.g., selfsoothing, fussing to parent, seeking comfort from the parent, problem-solving; Grolnick et al., 1998; Diener & Mangelsdorf, 1999) or examine the relation of security of attachment to children's abilities to self-regulate (Contreras et al., 2000). In addition, many measures of emotion regulation essentially tap the outcome of such regulation, for example, if the child shows neutral or positive emotion rather than distress (Carter et al., 1999) and/or is emotionally labile versus resilient (e.g., Shields & Cicchetti, 1997; Shields, Ryan, & Cicchetti, 2001). Such measures are most common in research on the relation of emotion regulation to adjustment. Despite this wide variety of measures that researchers use to assess emotion regulation, four primary methods are common: child self-report, parents' and teachers'

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reports, observational methods, and physiological measures. Each approach has strengths and weaknesses and, as we will discuss further in this section, the use of multiple methods of assessment usually is most desirable (for a more detailed discussion of specific measures of emotion regulation/control, see Eisenberg, Morris, & Spinrad, 2002). Another issue to consider in the assessment of emotionrelated regulation is the degree to which measures assess voluntary versus involuntary control. Many measures of emotion regulation tap emotionality and regulation, effortful control, or some combination of voluntary and involuntary control processes. In this section, we discuss the four common types of measures researchers typically use to assess emotion-related regulation and some of the issues involved in each type of method. We also discuss which specific measures tap voluntary versus involuntary control. We end this section by highlighting some examples of research that have included multiple methods of measurement, and we discuss the need for this type of research in the future. A. SELF-REPORT MEASURES

Self-report measures tap the individual's subjective cognitive experience of the emotion and the regulation process. Therefore, many investigators studying children old enough to report on emotion regulation rely on child self-report. The advantages of using self-report measures stem from the fact that many regulation processes and strategies are unobservable to others and, as a result, are difficult to measure. Self-report assessments tap the individual's own experience of regulation, which is beneficial in understanding how self-construction and awareness affect developmental outcomes. However, self-report measures have disadvantages because some individuals may not be aware of regulation strategies and processes that they engage in or may provide biased or self-serving responses. When child-reports are obtained, researchers sometimes focus more on the cognitive understanding of emotion rather than on actual emotion-related regulation (e.g., Dunn & Brown, 1994; Harris et al., 1981; Saami, 1979). Assessing children's understanding of emotion and the ways in which children discuss and learn about emotional processes is valuable; indeed, children's understanding of emotional processes is important in the development of emotion regulation and socioemotional competence (see Denham, 1999). However, researchers must be careful not to imply that an understanding of emotions and their regulation is emotion regulation. Some researchers have successfully examined children's and adolescents' selfreported emotion regulation in terms of their coping responses to hypothetical vignettes involving emotion and emotion management strategies (e.g., Band & Weisz, 1988; Saarni, 1997) and by using self-report questionnaire measures such as Rothbart's Early Adolescent Temperament Questionnaire (EATQ; Capaldi & Rothbart, 1992). When using hypothetical vignettes, investigators should consider

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that children may report the socially desirable response and not the response he or she would enact in real life. Indeed, Underwood, Coie, and Herbsman (1992) found that children nominated as aggressive by their peers did not differ from children classified as nonaggressive in their reactions to videotaped vignettes designed to elicit aggression-related emotions. Self-report measures of regulation typically tap components of voluntary control, specifically attention and inhibitory control (e.g., EATQ; Capaldi & Rothbart, 1992), or coping strategies. Research on coping has identified a long list of strategies for managing stress, which we see as similar to emotion-related regulation (e.g., cognitive restructuring, information seeking, problem-solving, emotional ventilation, acceptance, distraction, humor, social withdrawal, and denial; Compas, Connor, Saltzman, Thomsen, & Wadsworth, 2001). It often is unclear if coping reactions reflect effortful regulation or reactive control (e.g., involuntary inhibition that is seen as avoidance in stressful situations). Moreover, some of these measures likely assess emotionality (e.g., emotional ventilation) or outcomes of regulation. Investigators need to carefully consider what various measures of coping assess and how they relate to measures of emotion-related regulation. B. PARENT-AND TEACHER-REPORTMEASURES Utilizing adults' reports of children's emotional behavior has some advantages over using children's reports because adults are more skilled in answering questions related to emotions, and parents and teachers have the opportunity to observe children's emotion regulation over time and in a variety of social situations. Nonetheless, adults' reports are subject to certain biases, such as social desirability, and teachers may rate academically skilled children more positively on regulation and adjustment (Underwood, 1997). In addition, evidence on agreement among informants is mixed (e.g., Guthrie et al., 1997). Adult-report measures have scales that assess both voluntary and involuntary control. Researchers usually assess children's voluntary, effortful control using questionnaires that measure components of attention and inhibitory control [e.g., the Infant Behavior Questionnaire, Rothbart, 1981; the Toddler Behavior Assessment Questionnaire, Goldsmith, 1996; the Child Behavior Questionnaire, ages 3-7 (CBQ), Goldsmith & Rothbart, 1991, and Rothbart et al., 2001; the Early Adolescent Temperament Questionnaire, ages 9-15, Capaldi & Rothbart, 1992; and the Dimensions of Temperament Survey-Revised, Windle & Lerner, 1986]. Many of these measures have specific scales that assess attention focusing, attention shifting, and inhibitory control. Note that in early infancy, attentional control, such as shifting attention away from a distressing stimulus, may be involuntary; however, as already discussed, across the first years of life attentional control becomes more voluntary and effortful in nature. Consequently, the meaning of these measures may change with age.

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Parents and teachers can rate involuntary control (e.g., impulsivity and behavioral inhibition) on a variety of questionnaire measures as well (e.g., the impulsivity subscale of the CBQ; Rothbart et al., 2001). Impulsivity questionnaires typically measure the child's tendency to act without thinking (e.g., "sometimes interrupts others when they are speaking," item from the CBQ). Some parents likely cannot differentiate between inhibition due to effortful control and that due to temperamental behavioral inhibition; thus, adults' reports of inhibitory control sometimes may reflect a mix of effortful and reactive control. C. OBSERVATIONAL MEASURES

Many researchers assess emotion-related regulation observationally and attempt to quantify how children respond and behave emotionally in real-life situations. Infancy researchers often evaluate emotion regulation processes observationally (Underwood, 1997), whereas observational assessments are used somewhat less frequently in studies of older children, likely due to their cost (see, however, Eisenberg, Cumberland et al., 2001; Kochanska et al., 2000; Guthrie et al., 1997; Hubbard & Coie, 1994; Mischel, 1974, 2000). Observational research provides a relatively unbiased, objective assessment of emotion-related regulation, typically standardized across situations. This method allows for experts trained in observing emotion and related processes to quantify the behaviors that they observe. Despite the strength of this method, not all facets of emotion-related regulation are observable (e.g., cognitive reframing when one is alone), and results may not always generalize to outside of the laboratory or structured settings. Some researchers using observational methods to examine emotion regulation focus primarily on the intensity or duration of physical expression of emotions (facial and vocal expressions) as well as behavioral expressions of emotions (Thompson, 1994). Such measures actually assess emotionality and regulation only indirectly. However, some researchers have examined emotion regulation strategies (behaviors that help in the modulation of emotions) and their role in emotional control, such as self-soothing or distracting oneself with an activity (e.g., Grolnick, Bridges, & Connell, 1996; Mischel, 1974). Facial/gestural masking of emotion can also be considered a form of emotion regulation (Saarni, 1984), although it sometimes is difficult to know if a child is masking, or does not experience, emotion. Observational measures that assess voluntary control usually tap inhibitory control and/or attentional regulation (Eisenberg, Guthrie, et al., 1997; Mischel & Baker, 1972; Mischel & Ebbesen, 1970; Reed et al., 1984; also see Krueger, Caspi, Moffitt, White & Stouthamer-Loeber, 1996). Observational measures of attentional effortful control have primarily focused on infants' and children's persistence during difficult or boring tasks, their shifting attention to objects/activities

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when delaying gratification or dealing with a frustrating situation (Bridges et al., 1997; Diener & Mangelsdorf, 1999; Shoda, Mischel, & Peake, 1990), or their ability to resolve conflicts of attention between simultaneous stimulus events (e.g., Lab-TAB; Goldsmith & Rothbart, 1999; Grolnick, Bridges, & Connell, 1996). It should be noted that measuring components of attention may also tap into involuntary processes. For example, rumination--focusing on recurring thoughts---could be viewed as an involuntary attentional focusing that cannot be easily modified or controlled (Eisenberg, Morris, Spinrad, 2002). Unfortunately, such processes have not been explored in children. Kochanska and colleagues (Kochanska, Murray, & Coy, 1997; Kochanska et al., 2000) have created batteries of observational tasks to specifically measure inhibitory control in toddlers and children, and other researchers have used similar tasks to assess effortful inhibitory control in older children (Eisenberg, Cumberland, et al., 2001; Olson, Schilling, & Bates, 1999; White, Moffitt, Caspi, Bartusch, Needles, Stouthamer-Loeber, 1994). Observational measures of inhibitory control tend to assess toddlers' and children's ability to comply with demands (e.g., to clean up), inhibit motor behavior, suppress or initiate activities, or delay gratification. Tasks designed to measure children's control of facial disappointment when it is socially inappropriate probably also tap effortful control, at least to some degree. During these disappointment tasks, children's emotional responses to an unattractive prize are coded (Cole, Zahn-Waxler, & Smith, 1994; Eisenberg, Cumberland, et al., 2001; Saarni, 1984). Note that in certain observational tasks, however, children may appear regulated because they are overly inhibited (high in involuntary control) in the experimental context. Moreover, if a task involves a reward (e.g., as in delay or gratification tasks), short response times likely reflect involuntary reward-related approach (impulsive) tendencies as well as low effortful control. Observational tasks that involve cheating (e.g., Eisenberg, Cumberland, et al., 2001; Spinrad et al., 1999) can also be viewed as measuring low involuntary control (or high impulsivity due to BAS reward-dominance responding), although they also may tap externalizing behavior problems. Some observational measures that tap involuntary control specifically assess behavioral inhibition. For example, Kagan (1997) has measured infants' behavioral and emotional responses in stressful and/or novel situations (e.g., situations involving brightly colored toys moving back in forth in front of the face, tape recordings of voices speaking short sentences, and cotton swabs dipped in dilute butyl alcohol placed under infants' noses). Kagan and colleagues found that infants who were highly reactive, motorically active and crying, during these situations were more likely to be categorized as fearful and behaviorally inhibited at 14 and at 21 months, compared to the low reactive infants (Kagan, 1994, 1997). Thus, temperamental components of involuntary control such as behavioral inhibition likely can be assessed early on in development. Another measure that may partly assess involuntary control is reward dominance games. For example, in one such game researchers have children play a computer

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card game in which children win (reward) or lose (punishment) nickels; children can choose to discontinue the game at any time. Shortly into the game, the ratio of punishments to rewards increases steadily and the number of trials that children play before quitting the game is counted. This assessment has been viewed as a marker of impulsivity or reward dominance and has been associated with children's delinquent behavior (White et al., 1994) and externalizing problems (for children without comorbid anxiety problems; e.g., O'Brien & Frick, 1996). Such measures, however, might also involve the ability to integrate various inputs using executive attentional capacities (Patterson & Newman, 1993), thus tapping into executive (effortful) attentional functioning as well. D. PHYSIOLOGICAL MEASURES

A major component of emotionality and emotion modulation consists of the underlying physiological processes involved in arousal and its regulation (e.g., Porges, Doussard-Roosevelt, & Maiti, 1994). Researchers examining autonomic correlates of emotion regulation have focused primarily on the parasympathetic branch, which conserves and maintains bodily resources and works to regulate and decrease emotional arousal. Research on parasympathetic activation has relied primarily on indices of cardiac activity. Respiratory sinus arrhythmia (RSA), heart rate variability that occurs at the frequency of breathing, is thought to be a good estimate of parasympathetic influence (although some heart rate measures believed to tap parasympathetic functioning, often called vagal tone, have not actually measured respiration). Baseline cardiac vagal tone or RSA has been associated with differences in infants' appropriate emotional reactivity (Porter, Porges, & Marshall, 1988; Stifter & Fox, 1990; Stifter, Fox & Porges, 1989) as well as with the ability to adapt to a new situation in preschool (Fox & Field, 1989). Moreover, high RSA has been found to relate to infants' attentional abilities (Porges, 1991; Richards, 1987; Stifter et al., 1989). In addition, effortful, or voluntary, control has been linked to prefrontal cortical responding (Casey et al., 1997). Thus, in the future, brain imagining techniques are likely to become more common measures of emotion regulation. Furthermore, initial evidence indicates that asymmetries in activation of the frontal cortical lobes are linked to approach versus withdrawal (Tomarken & Keener, 1998); thus brain waves (EEG) may be associated with effortful and/or reactive approach or inhibition systems (Harmon-Jones & Sigelman, 2001). E. MULTIMETHOD APPROACHES

As mentioned previously, multiple methods of assessment provide the most thorough assessment of the regulation process, allowing for examination across multiple contexts and domains of regulation (e.g., physiological indicators and teachers' reports). A trend in psychological research is the examination of constructs using

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latent variables with multiple indicators of a construct in structural equation modeling. With this method, for example, measures derived from different methods or raters can (if they overlap) contribute to a single measure of the construct that reflects some contribution from each actual measure. A number of researchers have successfully examined emotion-related regulation using multiple methods. For example, Kochanska et al. (2000) used observational and parent-report measures to assess children's effortful control and found that the two methods converged, but primarily for data collected at the same point in time. Eisenberg and colleagues have used parents' and teachers' reports of emotion regulation in conjunction with observational tasks in multiple studies (e.g., Eisenberg, Fabes, et al., 2000; Eisenberg, Gershoff, et al., 2001) and have found that observational measures of emotion regulation are associated with adults' reports of the construct and that they, in combination, are related to both socialization and children's socioemotional development (e.g., adjustment, social competence). Moreover, Mezzacappa, Kindlon, and Earls (Kindlon, Mezzacappa, & Earls, 1995; Mezzacappa et al., 1998; Mezzacappa, Kindlon, & Earls, 1999) have used a variety of behavioral measures of regulation to examine different aspects of control and relate them to adjustment. Despite the benefits of a multimethod approach, such multimethod studies are not the norm (likely because of the difficulty in obtaining data from a variety of sources and the uncertainty in correspondence among methods), and there are few data on the relation between physiological indicators of emotion regulation and questionnaires attempting to assess similar processes.

VI. Relation of Emotion-Relevant Regulation to Quality of Social Functioning A central question in work on emotional regulation is whether it predicts important social and emotional outcomes for children. The simple answer is yes. However, relations vary somewhat depending on whether the measure of control actually taps effortful regulation or passive, less voluntary, control. A. THEORETICALEXPECTATIONS If one considers the difference between effortful control--which we view as regulationmand less voluntary types of control, what pattern of associations would we expect? Consistent with the distinction between overcontrolled and undercontrolled individuals in Block and Block's (1980) classic work on ego control, several groups of personality researchers have identified groups of overcontrolled, undercontrolled, and optimally regulated or resilient personality types in samples of children or adolescents (e.g., Asendorpf & van Aken, 1999; Hart, Hoffman, Edelstein, & Keller, 1997; Newman, Caspi, Moffitt, & Silva, 1997; Robins, John,

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Caspi, Moffitt, & Stouthamer-Loeber, 1996). However, measures of social functioning, with relatively little reference to emotion-related regulation, generally have been used to sort children into these groups. Nonetheless, it has been assumed that different aspects of control, as well as different styles of coping, are associated with overcontrolled, undercontrolled, and resilient personality styles. Eisenberg and her colleagues (Eisenberg, 2002; Eisenberg & Fabes, 1992; Eisenberg, Fabes, et al., 2000) have hypothesized that overcontrolled children are not really overregulated or effortfully overcontrolled--they are overly inhibited. Specifically, such children are hypothesized to be high in involuntary behavioral inhibition (e.g., inhibition to novelty, rigidity of behavior; high BIS responding); low to average in the abilities to effortfully inhibit or activate behavior as needed; low in voluntary, effortful attentional regulation; and low in those instrumental, problem-focused or engagement coping strategies that involve regulated approach behaviors. Thus these individuals are seen as somewhat low in effortful regulation--especially attentional regulation that relates to the control of internal emotional states (e.g., fear, anxiety)--and high in involuntary or reactive control. They are expected to be prone to internalizing problems (social withdrawal, anxiety/depression), especially if they are predisposed to negative emotionality. In contrast, undercontrolled individuals are predicted to be low in all types of effortful regulation, including effortful inhibitory, attentional, and activational control, as well as low in adaptive problem-focused and planful proactive coping behaviors. They also are hypothesized to be low in involuntary inhibition (BIS) and high in reactive approach (BAS) tendencies--that is, they are expected to be low in behavioral inhibition and to be impulsive. In general, people with this style of control are predicted to be relatively low in social competence and prone to externalizing behavior problems. Finally, optimally regulated individuals are hypothesized to be high in various modes of adaptive effortful regulation; because effortful control is used as needed, one is unlikely to exercise too much effortful control. In regard to involuntary control, optimally regulated individuals are viewed as neither overcontrolled (high in behavioral inhibition) nor undercontrolled (very impulsive). Moreover, because of their attentional and behavioral regulation, they would be expected to be planful when needed and, thus, should be prone to constructive problem solving and proactive coping. They are expected to be well adjusted, socially competent, and resilient to stress. An important question is whether effortful and reactive modes of control relate differently to developmental outcomes; they are more meaningful constructs if they provide some differential prediction. If we are correct in our theorizing, one would expect externalizing problem behaviors such as aggression, defiance, stealing, and the like, to be linked to low effortful control and low reactive control or high impulsivity. Thus, both effortful and reactive control are expected to be negatively related to externalizing behavior. In contrast, internalizing problems such as social

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withdrawal and anxiety would be expected to correlate with relatively low effortful control (especially attentional control) and with high reactive inhibition (and/or low impulsivity). In brief, although measures of impulsivity and effortful control tend to be substantially negatively related, we would expect them both to be negatively related to internalizing problem behavior and to provide some unique prediction of adjustment problems (or social competence). B. EMPIRICALDATA Empirical findings generally provide support for the role of control/regulation in the quality of children's social functioning, including their social competence, adjustment, and prosocial behavior (e.g., Caspi, 2000; Eisenberg, Fabes, et al., 2000; Eisenberg et al., 1996; Eisenberg, Pidada, & Liew, 2002; Kochanska, Coy, Tjebkes, & Husarek, 1998; Kochanska et al., 1997; Pulkkinen, 1982; Pulkkinen & Hamalainen, 1995; Rothbart, Ahadi, & Hershey, 1994; Rothbart & Bates, 1998). For example, even at 12 months of age, children who score higher on duration of orienting and on soothability (which likely partly reflects regulation) are more compliant, prosocial with peers, and/or empathic (Carter et al., 1999). Kochanska et al. (1997; Kochanska, Murray, Jacques, Koenig, & Vandegeest, 1996) found that behavioral measures of inhibitory control and parents' reports of this construct during the toddler, preschool, and early school years predicted internalized compliance, moral behavior (low cheating), rule-abiding behavior, and/or low selfish/antisocial solutions to hypothetical dilemmas in early childhood. Similar measures of effortful control at 22 and 33 months of age predicted lower anger at 33 months of age (Kochanska et al., 2000). Moreover, children's abilities to delay gratification have been found by Mischel and colleagues (Ayduk et al., 2000; Mischel, 2000; Mischel, Shoda, & Peake, 1988; Mischel, Shoda, & Rodriguez, 1989) to predict a host of positive outcomes 1 and 2 decades later in early adolescence and early adulthood, including academic and social competence, self-worth, and the ability to cope with frustration and stress (also see Lengua, West, & Sandier, 1998). 1. Externalizing Problems

Findings are clearer and more abundant for the prediction of externalizing (in comparison to internalizing) problems from measures of regulation or control. For example, low scores on behavioral measures of delay of gratification, stop-go tasks of inhibitory control, persistence tasks, and performance on a Stroop task, as well as adults' reports of low attentional control, low ego control, and high impulsivity, have all predicted externalizing problems (e.g., Krueger et al., 1996; Mezzacappa et al., 1999; Olson et al., 1999; Oosterlaan & Sergeant, 1996; see Eisenberg, Fabes, et al., 2000, for a review). In our own longitudinal work, for example, teachers' reports of 4- to 6-yearolds' attentional control (i.e., attention shifting and focusing) predicted children's

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social competence (a composite of socially appropriate behavior, popularity, and low aggression and disruptive behavior) at school 2, 4, and 6 years later. When the children were aged 4-6, attentional regulation also predicted peers' reports of boys' sociometric status and children's use of nonabusive language to deal with anger/frustration in real-life social interactionsmfindings that supported the validity of the adults' ratings of attentional control. When the children were 6 to 8-years-old, mothers' and teachers' reports of attentional and behavioral effortful regulation, combined with low impulsivity, also predicted socially competent behavior at school and low levels of parent- rated externalizing problems concurrently and 2 and 4 years later. Significant relations often were obtained within a given context (school or home), albeit not solely within reporter (e.g., findings often were across teachers or across parents). Some of the correlations were relatively substantial, that is, .40 or higher (Eisenberg, Fabes, et al., 1995, 1997; Murphy, Shepard, Eisenberg, & Fabes, 2001). In a study with young school-aged children, Eisenberg, Cumberland, et al. (2001) explicitly examined the relations of different types of control to children's externalizing problems. Specifically, they compared children with at least borderline levels of externalizing (with or without comorbid internalizing problem behaviors) to nondisordered children. The externalizing group was lower than nondisordered children on parents' (usually mothers') and teachers' reports of effortful attention shifting, attention focusing, and inhibitory control, and higher in reported impulsivity. These findings generally held across reporters; for example, when parents' reports of regulation or impulsivity were correlated with teachers' or fathers' reports of externalizing problems. Children with externalizing problems also were lower than nondisordered children on behavioral measures of control/regulation such as sitting still when asked and persisting on a task (rather than cheating). Children who were comorbid (i.e., were high in internalizing and externalizing problems) did not differ consistently from children who were high only on externalizing problems in regulation or impulsivity (Eisenberg, Cumberland, et al., 2001). Early problems with regulation also predict later adjustment and social competence. In addition to Mischel's work, Caspi and Moffitt in New Zealand (Caspi, 2000) and Pulkinnen in Finland (Pulkkinen & Hamalainen, 1995), for example, have found that the combination of low regulation and high dispositional negative emotionality in childhood predicts problem behaviors, delinquency, and other psychological and social problems in early adulthood. However, researchers seldom have controlled for earlier levels of adjustment or social competence when predicting later levels of these variables. Thus even tentative conclusions about causality cannot be drawn from most data. In one of the few studies in which investigators attempted to control for initial level of externalizing problems, Colder and Stice (1998) found that self-reported temperamental impulsivity did not predict high school seniors' delinquency across the school year when delinquency at the initial assessment was statistically

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controlled. However, reported impulsivity was highly consistent over the school year, which could account for the lack of an effect. In contrast, in a study with young school-aged children, Eisenberg, Guthrie, et al. (2000) found an association between regulation/control and externalizing problem behavior over 2 years, even when controlling for the initial level of externalizing problems. Specifically, they used structural equation modeling to test the prediction of externalizing problems from both attentional control (attention shifting and focusing) and behavioral control (selected items from Block and Block's Q-sort measure of ego control). At the 2-year follow-up, both attentional and behavioral regulation had unique effects on externalizing problem behavior, even when the across-time relations for problem behavior and for regulation were taken into account. Thus, children's regulation predicted a decline in externalizing problems over time, even when controlling for early levels of these variables. Moreover, the relation of attentional control to externalizing problems was moderated: this relation was significant primarily for children high in negative emotionality. 2. Internalizing Problems

Recall that internalizing children were also predicted to be low in effortful control, especially attentional control (which is believed to be involved in regulating negative emotions such as anxiety). However, in contrast to externalizing children, children with internalizing problems were hypothesized to be particularly prone to behavioral inhibition and low impulsivity--that is, to be high in the reactive aspects of control and low in approach tendencies. Evidence increasingly suggests that young children who are prone to behavioral inhibition or reactive control--who are constrained when dealing with novel or stressful situations--are prone to develop internalizing problems such as anxiety over time (Biederman et al., 1990; Kagan, Snidman, Zentner, & Person, 1999; Rosenbaum et al., 1993; see Rothbart & Bates, 1998). Also, children with internalizing problems are low in impulsivity and effortful control. In a study discussed previously (Eisenberg, Cumberland, et al., 2001), children who scored at least borderline in regard to internalizing problems (on Achenbach's, 1991, measures) and were not comorbid for externalizing problems were lower than nondisordered children in effortful attention shifting or focusing, and higher than externalizing children on these aspects of effortful control. However, although internalizers were consistently higher than externalizers in adult-rated inhibitory control (recall that inhibitory control is not the same as behavioral inhibition), they did not differ much from nondisordered children on these measures. Internalizers, in comparison to externalizers, also tended to express less negative emotion when disappointed (an index of the behavioral regulation of facial affect) and were somewhat more persistent when working on a task toward a reward; they were only slightly lower than control children on these tasks. In regard to reactive control, internalizers were lower than both nondisordered and externalizing children in impulsivity.

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Thus, internalizing children were quite high in reactive control (as tapped by low impulsivity) but were low in effortful attentional control. In brief, effortful and reactive control appear to differ functionally in their relations to internalizing problems, and effortful inhibitory and attentional control did not relate to the same degree to internalizing problems (deficits in the latter, but not the former, were linked to internalizing problems.) Thus, control processes apparently predict adjustment, although it is important to consider whether the control is effortful or not and to consider the type of effortful control.

VII. Relations of Dispositional Resiliency to Effortful and Reactive Control and Socioemotional Functioning A construct that is related to effortful regulation, but is not the same, is personality resiliency. Block and Block (1980) defined ego resiliency as "the dynamic capacity of an individual to modify his/her modal level of ego-control, in either direction, as a function of the demand characteristics of the environmental context" (p. 48). According to Block and Block, high resilience involves resourceful adaptation to changing circumstances and flexible use of problem-solving strategies, whereas low resilience involves little adaptive flexibility, an inability to respond to changing circumstances, the tendency to perseverate or become disorganized when dealing with change or stress, and difficulty recouping after traumatic experiences. The construct of "ego control" in the Blocks' definition of resiliency likely includes both effortful control and less voluntary modes of control. Specifically, ego control refers to the "threshold or operating characteristic of an individual with regard to the expression or containment of impulses, feelings, and desires" (Block & Block, 1980, p. 43). It varies on a continuum from ego overcontrol to ego undercontrol. Ego undercontrol involves insufficient modulation of impulses, the inability to delay gratification, immediate and direct expression of motivations and affect, and vulnerability to environmental distracters. Thus, ego undercontrol seems to involve impulsive approach tendencies and insufficient B IS tendencies or effortful control to rein in these impulsive tendencies. In contrast, ego overcontrol refers to the containment of impulses, delay of gratification, inhibition of actions and affect, and insulation from environmental distracters. Moderately high ego control may involve effortful regulation, but extreme levels of ego overcontrol likely are due to strong B IS tendencies as reflected in behavioral inhibition (e.g., constrained, fearful, and rigid behavior in novel and stressful circumstances). The concept of resiliency is relevant to the discussion of emotion-related regulation because resilient people seem to be optimally regulated and controlled. They can adapt well to changes in circumstances and cope well with stress. Thus, at least two interesting questions can be asked: (1) what is the relation

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of dispositional resiliency to various types of control/regulation, and (2) does dispositional resiliency play a role in the association between regulation and adaptive social functioning? A. THE RELATIONOF RESILIENCYTO REGULATION/CONTROL In regard to the relation of resiliency to various aspects of control, Block and Block (1980) believed that ego control and resilience were statistically independent, although they defined resiliency in relation to ego control. They also noted that "extreme placement at either end of the ego-control continuum implies a constancy in mode of behavior that, given a varying world, can be expected to be adaptively dysfunctional" (p. 44). Eisenberg and Fabes (1992) argued that this implies a quadratic relation between the constructs of ego control and resiliency. In regard to the notion of resiliency, Block and Kremen asserted that "the human goal is to be as undercontrolled as possible and as overcontrolled as necessary. When one is more undercontrolled than is adaptively effective or more overcontrolled than is adaptively required, one is not resilient" (1996, p. 351). If they are correct, resiliency would be expected to relate not only to the capacity to be spontaneous, but also to the ability to effortfully inhibit or control that spontaneity when necessary. In addition, resilient people would be expected to be low in involuntary behavioral inhibition and other expressions of relatively extreme (rather than moderate) BIS tendencies. Some initial data are consistent with these expectations. In three data sets we have obtained evidence that aspects of effortful control are positively related to resiliency. Eisenberg, Guthrie, et al. (1997; Eisenberg, Fabes, et al., 2000) found that the abilities to effortfully regulate attention (with attention shifting and focusing) were positively related to adults' ratings of resiliency in elementary school children at two points in time. The measure of resiliency was with items selected by experts from the Blocks' Q-sort that were rated as reflecting resiliency by experts in the Blocks' work (e.g., "Freezes up when things are stressful, or else keeps doing the same thing over and over again." and "Can bounce back or recover after a stressful or bad experience."). Similarly, in a second longitudinal data set with young school children (and a 2-year follow-up), we have found positive linear relations between effortful control (including attention shifting, focusing, and inhibitory control) and resiliency (Eisenberg, Spinrad, & Morris, 2002). Finally, this relation was replicated using similar measures of effortful control (and a behavioral index of persistence) and resiliency (Cumberland, Eisenberg, & Reiser, 2002). In the latter two samples, additional expert ratings were used to ensure that the items reflected resiliency rather than social competence, simple emotionality, or problem behaviors. Thus, considerable evidence indicates that children's effortful regulation is associated with resiliency. This is logical because effortful control can, by definition, be turned on and off as needed. Some evidence indicates that relatively high impulsivity, which likely partly reflects low BIS responding (and low levels of involuntary behavioral inhibition),

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is positively related to resiliency, although this relation may be strongest in young children. Cumberland et al. (2001) found that impulsivity, as well as effortful control, uniquely (and positively) predicted resiliency in a structural equation model. Furthermore, correlations between resiliency and impulsivity were examined in a sample selected to include children with at least borderline levels of externalizing and/or internalizing as well as nondisordered children (4.5 to nearly 8 years later). Relations between impulsivity and resiliency at this age were modestly positive but significant. Two years later, these correlations were weaker and at best marginally significant. At both ages, however, the relations between resiliency and teachers' (but not parents') reports of impulsivity were quadratic: children at the mean level of impulsivity were more resilient than those low in impulsivity, and children average and high in impulsivity were rated similarly on resiliency (Eisenberg, Spinrad, & Morris, 2002). Thus, resilient children, at least in the early school years, seem to be at least moderate in impulsivity and not overly controlled. B. RESILIENCY AS A MEDIATOR OF THE RELATION OF EFFORTFUL ATTENTIONAL CONTROL TO SOCIAL COMPETENCE

If resiliency is fostered by flexible, optimal regulation/control, it should mediate the relation between effortful control and social competence. Eisenberg and colleagues (Eisenberg, Guthrie, et al., 1997; Eisenberg, Fabes, et al., 2000) examined this mediated relation at two assessments in a longitudinal study. When the children were in kindergarten to third grade, parents and teachers reported on children's socially appropriate behavior and popularity, and peers reported on one another's sociometric status and prosocial tendencies. The primary caregiving parent (usually mothers) and teachers also rated children's attention shifting and focusing, ego control (i.e., probably a mix of effortful and reactive control), and resiliency. In addition, children played a game in which their persistence (vs cheating or off-task behavior) was assessed as an index of behavioral regulation. Two years later most of the same measures were administered again. We then examined the pattern of relations at both time periods, controlling for initial levels of a variable when looking at interrelations at the later assessment. We expected regulation to predict resiliency, which in turn was hypothesized to predict social functioning--popularity with peers and socially appropriate/prosocial behavior. Using structural equation modeling, separate latent constructs were obtained for attentional regulation and behavioral control, and each provided unique predictions of social competence at both assessments. The relations of attentional (i.e., emotional) controlma clear example of effortful regulation--to social status and socially appropriate behavior were mediated by resiliency at both assessments. Thus, children who could regulate their attention were rated as more resilient to stress and, perhaps as a consequence, were better liked by peers and viewed as being more socially appropriate or prosocial by others.

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The effects of behavioral control on social functioning were not mediated by resiliency. Rather, individual differences in behavioral control were directly related to socially appropriate behavior (but not social status). Furthermore, at both ages, this direct effect held only for children high in dispositional negative emotionality. Such children are especially likely to express negative emotions, perhaps because they have more frequent and intense emotions to manage, and lack of control of these emotions is likely to undermine their social behavior. At the first assessment only, the relation between adults' ratings of behavioral control and resiliency was quadratic (with moderate control related most or as well as high control to resiliency), suggesting that the measure of ego control had tapped overcontrol as well as effortful control. This is quite possible because the high end of Block and Block's construct of ego control reflects rigid overcontrol whereas the low end taps impulsivity (and the moderately high range likely measures a mix of effortful control and involuntary behavioral inhibition). Indeed, Derryberry and Rothbart (1997) suggested that Block and Block's construct of ego control taps primarily involuntary or reactive control (e.g., behavioral inhibition). In additional analyses, we found that attentional and behavioral regulation also predicted low levels of externalizing problem behavior, but resiliency did not mediate these negative relations (Eisenberg, Guthrie, et al., 2000) as it did for social competence (Eisenberg, Fabes, et al., 2000). The spontaneity associated with resiliency is also linked to impulsivity (which is positively associated with externalizing problems), so a linear negative relation between resiliency and externalizing problems might not be predicted. However, a negative relation between internalizing problems and resiliency is plausible. Children who are overcontrolled likely are especially limited in their spontaneity and ability to cope and rebound from stress. Recall that impulsivity has been associated with resiliency in early childhood and that overcontrolled children are low in impulsivity. Indeed, we have initial evidence of a relation between low resiliency and internalizing problems in our high-risk sample of children (Eisenberg, Cumberland, et al., 2001). In future work, we will examine whether the relation of effortful control to low internalizing problems is mediated by resiliency. We expect children who are able to control their attention and behavior voluntarily to be moderately prone to resiliency, which in turn would be expected to relate negatively to internalizing problems such as anxiety/depression and social withdrawal. In summary, the construct of resiliency is linked to optimal regulation in complex ways. Indeed, personality resiliency appears to be the outcome of optimal regulation--that is, an outcome of high levels of effortful control accompanied by flexibility and the capacity for spontaneity when it is not maladaptive. Thus, the construct of resiliency may help to explain how voluntary and involuntary aspects of control jointly affect adjustment and positive social functioning.

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VIII. Summary and Future Directions Initial findings suggest that the distinction between effortful regulation and reactive, less voluntary control/approach processes is useful and improves prediction of developmental outcomes. If children cannot voluntarily control their motivation, attention, and actions, they likely will have difficulty adapting to changing circumstances in flexible and appropriate ways. Moreover, because effortful regulation likely is more easily modified through learning than is the relaxing of behavioral inhibition or elimination of reward-related approach tendencies, distinctions among types of control/regulation have important implications for developing intervention and prevention programs. Despite the seeming conceptual relevance of the effortful versus the reactive regulation/control distinction, in many studies it is unclear what aspects of emotionrelated regulation are being assessed--voluntary or involuntary. Furthermore, children may appear to be well regulated when they simply are behaviorally inhibited in a stressful or novel situation or because they are less likely than their peers to experience emotional arousal. A daunting task for researchers is to differentiate between levels of emotional arousal and emotion regulation; children who differ in emotionality will tend to look and act differently even if they exert the same degree of emotion-related regulation. In addition, the field will benefit from research attempting to identify and develop measures that tap different aspects of regulation/control. Some evidence indicates that measures of reactive control are related to measures of effortful control (or the lack thereof). For example, adults' reports of children's impulsivity tend to be negatively related to their reports of inhibitory control (e.g., Eisenberg et aL, 1995) and effortful control is linked to low impulsivity in adulthood (Logan, Schachar, & Tannock, 1997). These relations are not surprising, however, because effortful regulation likely can be used to inhibit or otherwise modify the expression of reactive tendencies such as impulsivity. An important issue that has received little attention is the way in which the relation between effortful and reactive control changes with age. Because effortful control increases with age, as does the knowledge of socially appropriate and adaptive behavior, we predict that children increasingly use effortful control to manage overt manifestations of the tendencies toward reactive impulsivity or behavioral inhibition. Moreover, individual differences in effortful control may affect the degree to which children high in reactive control learn to control the expression of reactive tendencies. Intelligent and socially skilled children who are behaviorally inhibited become less so with age (Asendorpf, 1994); it is quite likely that these socially skilled children are relatively high in effortful control. The interconnections between effortful and reactive control, and how they change with age, is an issue that merits greater attention.

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Because effortful and reactive control appear to have neurological correlates, research in the neurosciences can contribute to an understanding of the interconnections between these two types of control. The work on executive attention (e.g., Posner & DiGirolamo, 2000), for example, is providing valuable information on effortful attentional control. As knowledge on neurological functioning and interconnections among various aspects of the brain expands, developmentalists are likely to learn more about both emotion and the complex processes involved in its regulation. Most of the research on emotion-related regulation/control and its role in development has been conducted with infants and children. Yet adolescence is associated with increased negative emotion for some individuals as well as with increased levels of aggressive and antisocial behavior (Larson & Lampman-Petraitis, 1989; Moffitt, 1993). It will be fruitful to examine the relations of effortful and reactive control in childhood to adolescents' socioemotional status and to chart changes in the complex interplay of effortful and reactive processes during adolescence. In summary, emotion-related regulation and control are complex constructs that are difficult to assess. Perhaps more than for many other constructs, a multimethod approach is needed to advance our understanding of these constructs. Moreover, longitudinal research and research with children with dysfunctions in emotion regulation are needed to examine causal relations between emotion-related regulation/control and developmental outcomes. It also is important to examine the mediators and moderators of the effects of regulation/control as well as biological and environmental factors that contribute to the development of, and individual differences in, effortful regulation.

ACKNOWLEDGMENT Work on this chapter was supported by grants from the National Institutes of Mental Health (1 R01 MH 60838) and from the National Institute of Drug Abuse (DA 05227).

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Hoffner, C. (1993). Children's strategies for coping with stress: Blunting and monitoring. Motivation and Emotion, 17, 91-106. Hornik, R., Risenhoover, N., & Gunnar, M. (1987). The effects of maternal positive, neutral, and negative affect communications on infant responses to new toys. Child Development, 58, 937-944. Hubbard, J. A., & Coie, J. D. (1994). Emotional correlates of social competence in children's peer relationships. Merrill-Palmer Quarterly, 40, 1-20. Izard, C. E. (1977). Human Emotions. New York: Plenum. Kagan, J. (1994). On the nature of emotion. In N. A. Fox (Ed.), Monographs of the Societyfor Research in Child Development (Serial No. 240, Vol. 59, pp. 7-24). Chicago, IL: University of Chicago Press. Kagan, J. (1997). Temperament and the reactions to unfamiliarity. Child Development, 68, 139143. Kagan, J., Snidman, N., & Arcus, D. M. (1992). Initial reactions to unfamiliarity. Current Directions in Psychological Science, 1, 171-174. Kagan, J., Snidman, N., Zentner, M., & Person, E. (1999). Infant temperament and anxious symptoms in school age children. Development and Psychopathy, 11, 209-224. Kaye, K., & Fogel, A. (1980). The temporal structure of face-to-face communication between mothers and infants. Developmental Psychology, 16, 454-464. Kindlon, D., Mezzacappa, E., & Earls, E (1995). Psychometric properties of impulsivity measures: Temporal stability, validity, and factor structure. Journal of Child Psychology and Psychiatry and Allied Disciplines, 36, 645-661. Kliewer, W. (1991). Coping in middle childhood: Relations to competence, type A behavior, monitoring, blunting, and locus of control. Developmental Psychology, 27, 689-697. Kochanska, G., Coy, K. C., Tjebkes, T. L., & Husarek, S. J. (1998). Individual differences in emotionality in infancy. Child Development, 64, 375-390. Kochanska, G., Murray, K., & Coy, K. (1997). Inhibitory control as a contributor to conscience in childhood: From toddler to early school age. Child Development, 68, 263-277. Kochanska, G., Murray, K., & Harlan, E. T. (2000). Effortful control in early childhood: Continuity and change, antecedents, and implications for social development. Developmental Psychology, 36, 220-232. Kochanska, G., Murray, K., Jacques, T. Y., Koenig, A. L., & Vandegeest, K. A. (1996). Inhibitory control in young children and its role in emerging internalization. Child Development, 67, 490-507. Kopp, C. B. (1982). Antecedents of self-regulation: A developmental perspective. Developmental Psychology, 18, 199-214. Kopp, C. B. (1989). Regulation of distress and negative emotions: A developmental view. Developmental Psychology, 25, 343-354. Kopp, C. B. (1992). Emotional distress and control in young children. New Directions for Child Development, 55, 41-56. Kopp, C. B., & Neufeld, S. J. (2002). Emotional development during infancy. In R. Davidson, K. Scherer, & H. H. Goldsmith (Eds.), Handbook ofaffective sciences. Oxford, UK: Oxford University Press. In Press. Krueger, R. E, Caspi, A., Moffitt, T. E., White, J., & Stouthamer-Loeber, M. (1996). Delay of gratification, psychopathology, and personality: Is low self-control specific to externalizing problems? Journal of Personality, 64, 107-129. Larson, R., & Lampman-Petraitis, C. (1989). Daily emotional states as reported by children and adolescents. Child Development, 60, 1250-1260. Lazarus, R. S., & Folkman, S. (1984). Stress, appraisal, and coping. New York: Springer-Verlag. Lengua, L. J., West, S. G., & Sandier, I. N. (1998). Temperament as a predictor of symptomatology in children: Addressing contamination of measures. Child Development, 69, 164-181. Logan, G. D., Schachar, R. J., & Tannock, R. (1997). Impulsivity and inhibitory control. Psychological Science, 8, 60-64.

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MATERNAL SENSITIVITY AND ATTACHMENT IN A T Y P I C A L G R O U P S

L. Beckwith 1, A. Rozga 2, and M. Sigman 2 1DEPARTMENT OF PEDIATRICS AND 2DEPARTMENT OF PSYCHIATRY UNIVERSITY OF CALIFORNIA AT LOS ANGELES LOS ANGELES, CALIFORNIA 90024

I. INTRODUCTION II. ORGANIZATION OF ATTACHMENT III. MATERNAL SENSITIVITY A. CONCEPTUAL ISSUES B. METHODOLOGICAL ISSUES C. ASSOCIATIONS WITH ATTACHMENT SECURITY IV. ROLE OF CHILD CHARACTERISTICS A. TEMPERAMENT IN TYPICALLY DEVELOPING CHILDREN B. ATYPICALLY DEVELOPING CHILDREN V. ATYPICAL GROUPS OF MOTHERS A. MOTHERS WHO ABUSE DRUGS AND ALCOHOL B. MATERNAL DEPRESSION VI. CONCLUSIONS REFERENCES

I. Introduction The parent-child relationship develops and is shaped by both parental and child characteristics. Although the burden of responsibility is on the adult, the child, from birth, behaves in ways that will elicit care from the parent. Infants are genetically programmed to behave in ways that promote and maintain contact with other humans. The infant, through discriminative crying, looking, and smiling, initiates, maintains, and responds to social interaction (Bell, 1981). Infants actively seek out stimulation, attend selectively, and show strong preferences among stimuli and for 231 ADVANCESIN CHILDDEVELOPMENT AND BEHAVIOR,VOL.30

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certain levels of stimulus change. Significant individual differences are noted. But what if the infant's initiations are seriously diminished, as occurs with very low birthweight preterm infants who are born very early (Goldberg & DiVitto, 1995)? Or what if the child's initiations and responses are deviant, as in the case of either Down syndrome or autistic children? Atypical children stress the parent-child relationship and show the degree to which parents can adapt to their child's characteristics. Further, the child's ability to elicit and maintain the power of its role in the relationship is highlighted in atypical groups and may provide insights on how individual differences among children mold parental behavior in normative groups. In this chapter we focus on children who have diagnosable conditions or disorders that were identified either at birth or during infancy. We consider infants born preterm, infants with serious medical conditions, children with Down syndrome, and children with autism. Similarly, we discuss children of mothers with identified, diagnosable psychopathology, including substance abuse and psychiatric disorders. In our review we consider to what degree, and how, members of the group achieve normative stages of development. Our approach can be contrasted with a model of human development in which risk factors, arising from both biological and social influences, shape the paths by which individuals become vulnerable or resistant to psychological problems and disorders. Risk factors are circumstances that increase the possibility of the occurrence, severity, duration, or frequency of later psychological disorders. Risks exist in multiple spheres: vulnerabilities in the child; psychological distress in the parent; and problems in family functioning arising from marital discord, divorce, alcohol or substance abuse, or stressful life events. In general, the greater the number of risk factors to which a child is exposed, the greater the likelihood of later mental health disturbance (Sameroff, Seifer, Zax, & Barocas, 1987). The link between risk factor and outcome is not specific. Most risks can be shown to increase the likelihood of a variety of later disturbances. Therefore, multiple paths, including different combinations of risk factors, can lead to the same outcome; and the same risk factor can result in multiple outcomes, dependent on the biological and social context of development. Although risk research and the study of atypical groups overlap, they differ conceptually and methodologically. Because risk does not determine outcome, some or many infants in a risk group will not develop a disorder at any time. Also, because a risk factor may cause a variety of later conditions, developmental pathways will be different even among those who share the same risk factor. In an atypical group, members tend to be more homogeneous at least in the psychological characteristics associated with the diagnosis or condition, enhancing the study of how altered infant or maternal characteristics affect the caregiving system. However, even in an atypical group, members vary in the intensity, extensiveness, and timing of their symptoms. Even in an atypical group, parents' and childrens'

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lives change; new experiences, supportive and traumatic, occur during childhood. New developmental achievements and integrations occur. Although we focus on the attachment function of the parent-child relationship, parents perform other functions that also serve developmental needs. They feed and maintain physical health. They may be playmate or teacher. They may organize the environment to be safe and developmentally stimulating and appropriate. They instruct and discipline, and they encourage or set limits to exploration. In those roles, they may act sensitively or insensitively, and in so doing, influence outcomes other than attachment. For example, maternal sensitivity among cocaine-addicted and PCP-addicted women encourages more organized play in their toddlers (Beckwith, Rodning, Norris, & Phillipsen, 1994). Sensitive mothers of premature infants increase the likelihood of their child having a higher IQ from age 2 years on, achieving in school, and perceiving themselves as socially and cognitively competent, regardless of actual cognitive competence (Beckwith & Cohen, 1989). The influence exists and is evident even when neonatal characteristics of brain structure shown in sleep-state organization (Beckwith & Parmelee, 1986) and visual attentiveness (Sigman, Cohen, Beckwith, Asarnow, & Parmelee, 1992) predispose for poorer functioning. These functions are important but in this chapter we consider only the attachment function. We seek to differentiate maternal and child contributions to the motherchild relationship, specifically as they affect maternal sensitivity and child security in atypical groups. Because of the increased variability in affective and cognitive functioning, and the overt deficits that are evident, atypical groups have the power to make more apparent the affective and cognitive skills required in each partner for a smooth attachment relationship. The chapter is organized as follows: we discuss the meaning and measurement of attachment security, the construct of maternal sensitivity, the characteristics of children that affect maternal sensitivity and attachment patterns in normally developing and atypical groups of children, and the dynamics underlying attachment patterns in offspring of atypical mothers. We end the chapter by reflecting on the implications for normal and atypical child development.

II. Organization of Attachment Human infants are biologically predisposed through evolution to form attachments to caregivers (Bowlby, 1969). Attachments may not occur only under exceedingly rare life conditions, where there is no stable person with which the infant can interact, such as in certain kinds of institutional rearing. In family-reared infants, the issue is not the presence of attachment, nor its strength. Rather, attachment theory identifies individual differences in attachment as categorical rather than as quantitative (Ainsworth, Blehar, Waters, & Wall, 1978; Bowlby, 1973). A

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central tenet of attachment theory from its inception has been that an infant's attachment develops within the context of, and is therefore strongly shaped by, early caregiver-child interactions (Bowlby, 1969; Belsky, 1999). Bowlby (1973) postulated that through a history of responsive care, infants evolve expectations of their caregiver's likely responsiveness to their signals of distress or need for comfort and contact; that is, the type of attachment they develop depends on the nature of the environments in which they are reared. Security of attachment was defined as a preferential desire for contact with the caregiver under conditions of threat or fear. Ainsworth developed the Strange Situation (Ainsworth et al., 1978) to examine infants' interactive behavior with their caregiver under conditions of mild to moderate stress of the attachment system. The Strange Situation introduces a laboratory situation that is unfamiliar, an unfamiliar adult who interacts with the child, and two brief separations from the mother. The reasoning is that the multiple increasing stressors will activate the infant's attachment system and that individual differences in the child's expectations of the caregiver will be revealed. Simultaneously, the Strange Situation demonstrates the infant's ability to balance the need for comfort and reassurance from the caregiver with exploration of the new environment. Ainsworth discovered and described three patterns that infants show in the Strange Situation. As explicated by Weinfeld, Sroufe, Egeland, and Carlson (1999), infants classified as secure use the caregiver as a secure base from which to explore the novel room and novel toys. The secure infant, in the presence of the caregiver, explores the toys, even while checking back with the caregiver or showing the toys to the caregiver. Upon separation, the infant's play becomes impoverished, and the infant may become overtly distressed. Upon reunion with the caregiver, a distressed secure infant will seek proximity or contact, will be comforted by the proximity or contact, and will eventually return to play. Secure infants who do not become overtly distressed during separation still respond to the caregiver's return by initiating interaction with a smile or vocalization. Infants classified as insecure avoidant also explore the toys in the presence of the caregiver, but the infant is unlikely to smile or vocalize or to show the toys to the caregiver except for instrumental assistance. Upon separation, the insecure avoidant infant is unlikely to become distressed when the stranger is present, although some distress may be shown when left completely alone. Upon reunion, the infant ignores, or looks or turns away, or moves past the caregiver without approaching. If picked up, the insecure avoidant infant does little to maintain the contact. Infants classified as insecure resistant do not seem to be able to use the caregiver as a secure base from which to explore. Their play may be limited and they may seek contact and proximity from the caregiver even before separation occurs. Upon separation, insecure resistant infants are likely to become quite distressed. Upon reunion they may seem to want proximity or contact with their caregiver but are not comforted by it. Some insecure resistant infants are oddly passive, crying but failing to actively seek contact; others seek contact, and then resist it with displays of anger.

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Among nonnormative groups, attachment patterns are seen that are themselves atypical and inconsistent with Ainsworth's original classifications. In normative or low-risk groups, 85-90% of children typically can be described by the original three categories (Main & Solomon, 1986; Sroufe & Waters, 1977), but with atypical groups, only 15-65% of children can be categorized as secure, insecure avoidant, or insecure resistant (Barnett, Buffer, & Vondra, 1999). In fact, four atypical attachment patterns have been identified in normative and atypical children: disorganized/disoriented (Main & Solomon, 1986, 1990), avoidant/ambivalent (Crittenden, 1985), unstable/avoidant (Lyons-Ruth, Connell, Zoll, & Stahl, 1987), and unclassified. We will focus on the disorganized/disoriented classification because most research has been done using that category, in part because it incorporates many features of the avoidant/ambivalent and unstable/avoidant classifications. Three overlapping levels of deviation from normative patterns of behavior in the Strange Situation are considered (Barnett & Vondra, 1999). First, at the level of organization of behavioral systems, secure children move smoothly in predictable sequences of approach and contact with the caregiver during distress, followed by relief, and then autonomous exploration and play. That is, the attachment, fearwariness, and exploration systems are coordinated. Children with atypical patterns may neither explore nor show attachment behavior to the caregiver, or they may show fear-wariness simultaneously with attachment. Second, at the level of behavioral interactions, secure children tend to show low levels of avoidance and resistance and to show positive affect by smiling at and vocalizing to the attachment person (Waters, Wippman, & Sroufe, 1979). Those who are not overtly distressed on separation from their caregiver tend to show low-to-moderate proximity-seeking and contact maintenance upon reunion, whereas those who get highly distressed tend to show high levels of proximity-seeking and contact maintenance. In contrast, children with atypical patterns may combine high separation distress with low proximity-seeking and contact maintenance upon reunion; or show low levels of avoidance and resistance but a marked absence of positive affect; or show high levels of both avoidance and resistance. Third, at the level of discrete behaviors, secure children never show dazed or fearful facial expressions, freezing or stilling, or coveting the mouth on the approach of the attachment person. Children who show these behaviors are atypical (Main & Solomon, 1986), even when the overall sequencing of behavioral systems or overall interactive pattern of behaviors are normative. However, these behaviors must be characteristic of the relationship; that is, they must occur with increased frequency around the caregiver and not simply reflect neurological or genetic impairments, such as motoric stilling in children with cerebral palsy or hand-flapping or self-stimulating behavior in autism (Atkinson et al., 1999; Main in Capps, Sigman, & Mundy, 1994; Pipp-Siegel, Siegel, & Dean, 1999). Disorganized early attachments are probably a marker of a process that may lead to later increased pathology (Sroufe, Carlson, Levy, & Egeland, 1999). In the few published studies, the disorganized/disoriented classification is associated with

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later social-emotional problems, particularly oppositional or hostile-aggressive behaviors (Lyons-Ruth, 1996). Furthermore, children with disorganized attachment patterns have been found to be more vulnerable to altered states of mind such as dissociation in young adulthood (Carlson, 1998). Although some investigators have questioned whether atypical attachment patterns are more unstable across time than typical attachment patterns (Lyons-Ruth, Repacholi, McLeod, & Silva, 1991; Vaughn, Waters, Egeland, & Sroufe, 1979), the evidence is inconsistent on the degree of stability for both typical and atypical attachment patterns. Some investigators find significant stability from infancy to adulthood of typical attachment patterns (77%, Hamilton, 2000; 72% Waters, Merrick, Treboux, & Crowell, 2000). Other investigators find significant change (50%) even from 12 to 18 months (Vondra, Hommerding, & Shaw, 1999), a degree of change that exceeds that reported in atypical children such as Down syndrome children (62% stability from 26 to 42 months, Atkinson et al., 1999) and maltreated children (66% stability from 12 to 18 months, Barnett, Ganiban, & Cicchetti, 1999). Changes in attachment organization clearly exist among all groups--typical and atypical. Such changes do not diminish their developmental significance. Changes are probably lawful and related to changes in the family environment or characteristics within the developing child. Although few studies exist that test the continuity and change in attachment organization in atypical groups, much could be learned in the future about the evolving nature, causes, and consequences of attachment patterns among atypical mothers and children.

III. Maternal Sensitivity A. CONCEPTUALISSUES In elucidating the antecedent conditions contributing to the development of secure attachment, Bowlby (1988) emphasized the infant's confidence in his mother's accessibility and responsiveness. Mary Ainsworth operationalized the construct of maternal sensitivity for empirical investigation and explicated its implication for the development of qualitative differences in infant-mother attachment (Ainsworth et al., 1978). Based on extensive naturalistic observations of mother-infant interactions throughout infants' 1st year of life, Ainsworth discovered that the different patterns of infant behavior observed in the Strange Situation arose from differential experiences of bids and responses within the caregiver-infant dyad during the preceding year and the resulting differential expectations regarding the availability and responsiveness of the caregiver. Specifically, Ainsworth operationalized the construct of maternal sensitivity as the mother's ability to perceive and accurately interpret her infant's signals and communications and to respond to these signals promptly and appropriately (Ainsworth et al., 1978).

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In developing her composite measure of maternal sensitivity, Ainsworth identified four core components that characterize the optimally sensitive mother: (1) awareness of the baby's signals, (2) accurate interpretation of these signals, including the ability to empathize with the infant, (3) appropriate response to these signals, particularly one that is developmentally appropriate, and (4) promptness of the response to the baby's signals (Ainsworth et al., 1978). Although Ainsworth's definition of sensitivity has been used widely (e.g., Bretherton, 1985; Isabella, 1993; Sroufe, 1988), other theorists have reconceptualized and extended sensitivity to include other aspects of maternal behavior (De Wolff & van IJzendoorn, 1997; see Nicholls & Kirkland, 1996, for a review of maternal sensitivity definitions). Consequently, definitions of sensitivity have varied in the relative emphasis placed on specific aspects of maternal behavior, in particular with respect to expressions of affect and emotion regulation. Researchers have emphasized the mother's affectionate behavior and expressions of positive affect and praise as important components of sensitivity (Biringen, 1998; Crawley & Spiker, 1983; Crittenden, 1981; Easterbrooks & Goldberg, 1984) as well as the importance of the mother's open acceptance of and response to a range of the child's affective signals (Biringen & Robinson, 1991; Cassidy, 1994). Emotion regulation is also central to Stem's (1983) notion of affective attunement, whereby the mother and infant become behaviorally coordinated through the matching of affect (see also Field, 1994). The mother's role is one of modulating the infant's optimal stimulation and arousal level by reading the infant's signals and matching the infant's behavior in form, intensity, and temporal features such as duration and rhythm, such that their affective behaviors become synchronized (Field, 1994; Stem, 1983). Thus a sensitive mother not only perceives the infant's affective signals but, more important, also accepts these signals and shares in them with her infant (Stem, 1985). Thus maternal sensitivity has been reconceptualized within the framework of emotional availability (Biringen & Robinson, 1991), focusing on the mother's emotional responsiveness and affective attunement to the infant's signals (Emde, 1980; Mahler, Pine, & Bergman, 1975). Central to the emotional availability framework is the mother's acceptance of a wide range of infant emotions, both positive and negative, rather than simply a focus on distress situations (Biringen & Robinson, 1991; Emde, 1980). The definition of maternal sensitivity within this framework subsumes qualifies of Ainsworth's original concept~mainly an accurate perception and interpretation of infant signals and a prompt and appropriate response to these signals~but augments them with an added focus on the role of affect and conflict regulation. Central to this reconceptualization is the open expression of a range of affect within the dyad, resulting in positive and appropriate emotional exchanges between the parent and the infant (Biringen & Robinson, 1991). Moreover, the mother's affect must be appropriate and genuine, such that there is congruence between the parent's verbal and nonverbal means of affective communication.

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Thus, this framework includes Ainsworth's focus on the mother's appropriate responsiveness to infant signals but within the broader context of emotion (Biringen & Robinson, 1991). Another range of studies has augmented the more traditional concepts of maternal sensitivity and the original emphasis on the quality of mother-infant behavioral bids and responses by emphasizing interactional synchrony, mainly rhythmic coordination and interpersonal contingency in mother-infant interactions (Belsky, Rovine, & Taylor, 1984; Brazelton, Koslowski, & Main, 1974; Cohn & Tronick, 1988; Isabella & Belsky, 1991; Isabella, Belsky, & yon Eye, 1989; Jaffe, Beebe, Feldstein, Crown, & Jasnow, 2001; Stern, 1974). Early research in this area emphasized mother-infant "dialogues," or reciprocal action exchanges, during face-to-face interactions. For example, Stern (1974) and Brazelton and colleagues (Brazelton et al., 1974) focus on the mother's ability to monitor her infant's cycles of attention and withdrawal and to effectively interweave her behavior with these patterns. Timing and rhythm are considered crucial to early mother-infant interactions; the mother plays the central role in maintaining synchronous interactions by mirroring the infant's behavior and adjusting the rhythm and tempo of her behavior to that of her infant. Moreover, research by Cohn and Tronick (1988) and Jaffe and colleagues (Jaffe et al., 2001) indicates that such mother-infant coordination is bidirectional: both interactional partners influence one another's behavior and thus also the direction of the interaction. In a similar vein, interactional synchrony, as conceptualized by researchers such as Belsky and Isabella (Belsky, Taylor, & Rovine, 1984; Isabella & Belsky, 1991; Isabella et al., 1989), is reflected in reciprocal patterns of interaction that result from co-occurrences of infant and maternal behavior. These patterns of reciprocity, though driven by mothers' contingent and stimulating responsiveness to infant vocalizations and bids for attention, are nonetheless inherently dyadic, relying as much on infants' initiations as on maternal responses (Belsky, Rovine, et al., 1984). Maternal sensitivity may also be viewed within the broader context of the attachment system and the parental behaviors associated with it, particularly the tendency to seek, comfort, and retrieve the child needing care when in frightening circumstances (Cassidy, 1999). Main and Hesse (1990) propose that maternal fear and maternal behaviors that are frightening to children, both derived from the mother's experience of unresolved loss and mourning over an attachment figure or from unresolved experiences of sexual or physical abuse, may be crucial features of maternal interactive behavior that promote the development of disorganized/disoriented infant attachment. The child is placed in a conflictual position of seeking comfort when frightened or distressed from a caregiver who herself is a source of fear. In line with this proposal, ratings of maternal frightening behavior, such as creeping up from behind the infant or incidents of physical and sexual abuse, as well as frightened behavior, such as seeking safety and comfort with the infant or viewing the infant as a source of alarm, appear to be associated with

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disorganized attachment (Lyons-Ruth, Bronfman, & Parsons, 1999; Schuengel, van IJzendoom, Bakermans-Kranenburg, & Bloom, 1998). Moreover, as ratings of maternal sensitivity are not associated with the disorganized attachment pattern (van IJzendoorn, Schuengel, & Bakermans-Kranenburg, 1999), maternal frightening/frightened behavior appears to be a distinct dimension of maternal behavior uniquely predictive of the disorganized/disoriented attachment pattern. Evidently when the caregiver acts in a way that frightens the child--behavior that is antithetical to his/her role as a secure base--this may have dire implications for the formation of a secure attachment relationship. In keeping with assertions of traditional attachment theory (Ainsworth et al., 1978; Bowlby, 1969), parental behaviors that institute proximity and comfort when the parent perceives the child to be in real or potential danger (Cassidy, 1999) are directly relevant to the attachment relationship and remain theoretically distinct from other domains of caregiving. Parental interactive behaviors associated with attachment, including maternal sensitivity, are therefore considered independent of other aspects of mothers' interactions with their infants, such as provision of structure, stimulation, and teaching style, or even meeting the infant's physiological needs (Bowlby, 1969). B. METHODOLOGICAL ISSUES

Just as maternal sensitivity has been redefined, a variety of distinct approaches have been developed to evaluate the quality of early infant-mother interaction. Ainsworth adopted the method of rating matemal behavior based on detailed narrative records of specific behaviors observed during naturalistic observations of mother-child interaction throughout the infant's 1st year of life. The resulting sensitivity rating scale (Ainsworth et al., 1978) provided a global evaluative rating of the mother's interactive style focusing on the aforementioned four core components characterizing the optimally sensitive mother. Although few researchers have conducted such extensive naturalistic observations, a number have followed Ainsworth's lead in rating maternal sensitivity based on observations of motherinfant interaction using qualitative rating scales (e.g., Biringen, 1998; Crawley & Spiker, 1983) that focus on variations in maternal sensitivity and responsiveness to infant signals and communications. Despite the ubiquity of rating-scale type assessments of maternal sensitivity, Pederson and Moran (1995) argue that such descriptive techniques are insensitive to important differences in mothers' behavior with their infants. Consequently, their Maternal Behavior Q-Sort (Pederson, Moran, Sitko, & Campbell, 1990) requires that the mother be observed in circumstances that force her to divide her attention between her infant and tasks imposed by the researchers; observers focus on those aspects of the interaction particularly relevant to attachment (Pederson & Moran, 1995). Maternal interactive behavior is rated by sorting behavioral descriptions

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tapping the mother's ability to perceive her infant's signals and the promptness and appropriateness of her response into a predefined number of piles ranging from least to most characteristic of the mother. The mother's rated sensitivity is the correlation between the observer's descriptive sort and a criterion sort that describes a prototypically sensitive mother. Qualitative rating scales also have been criticized because they may overemphasize particular aspects of sensitivity, such as warmth and supportiveness, over skill-related aspects of perceiving and appropriately responding to the infant's signals (Mangelsdorf, Gunnar, Kestenbaum, & Lang, 1990; Seifer & Schiller, 1995). Consequently, some researchers have focused on frequency counts, using precoded checklists to note the occurrence of theoretically specified indicators of sensitivity during observations of mother-child interaction (Beckwith & Cohen, 1984; Belsky, Taylor, et al., 1984; Isabella & Belsky, 1991; Isabella et al., 1989; Lewis & Feiring, 1989). The frequency of developmentally relevant indicators of sensitivity that emphasize prompt and appropriate responsiveness to infant cues (e.g., gaze, smile, fuss/cry, nondistress vocalization, gesture) are noted at prespecified time intervals (Beckwith & Cohen, 1984). Such time-sampled frequency counts allow researchers to note the co-occurrence of infant and mother behaviors--including direct maternal responses to infant cues or infant responses to maternal stimulation, and three-step contingent exchanges (e.g., infant vocalizes, mother gazes at infant and smiles, infant coos)----as well as the timing and frequency of maternal behavior following specific child behaviors and thus to capture synchronous mother-infant interactions. Moreover, researchers have also employed more microanalytic time-series analyses in assessing synchrony of mother-infant face-to-face and vocal interactions (e.g., Cohn, Campbell, & Ross, 1991; Cohn & Tronick, 1988; Jaffe et al., 2001), which yield separate indices of whether either partner's behavior is coordinated with the other. Surprisingly, when sensitivity is measured with behavior frequency approaches, intermediate levels of reciprocal interaction appear optimal for infant attachment security (e.g., Belsky, Taylor, et al., 1984; Jaffe et al., 2001; Lewis & Feiring, 1989; although see Beckwith & Cohen, 1984). On traditional rating scales, high ratings of maternal sensitivity have been associated with infant attachment security (Ainsworth et al., 1978). In contrast, with behavior frequency counts, high levels of maternal interactive involvement appear associated with avoidant attachments, whereas low levels of involvement are associated with resistant attachment. High rates of maternal responsivity may actually reflect a certain amount of maternal intrusiveness (e.g., bids for infant attention when the infant has withdrawn from the interaction), whereas low rates reflect failures on the mother's part to respond to infant vocalizations and gestures. Approaches used much less frequently include questionnaires (Izard, Haynes, Chisholm, & Baak, 1991) and interviews with the mother (Bretherton, Biringen, Ridgeway, Maslin, & Sherman, 1989). Bretherton and colleagues (Bretherton et al.,

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1989) interviewed mothers regarding a number of attachment-related issues, such as separation experiences, bed and nighttime experiences, autonomy-related negotiations, and intergenerational similarities and differences, then asked them to describe specific situations when answering these questions. A rating scale was developed to assess the mother's sensitivity and insight regarding her relationship with her child, particularly the consistency between the mother's general statements and the specific descriptions she provided. High scores reflect the mother's ability to convey the sense that she responds appropriately and sensitively to her child's communications and that she has a great deal of insight into her child's behavior and personality. c. ASSOCIATIONSWITHATTACHMENTSECURITY Despite the variety of definitions and methods of assessment, maternal sensitivity is a significant predictor of attachment security in normative samples. Consistent with Ainsworth's original work, ratings of maternal sensitivity throughout infants' 1st year of life are linked to subsequent attachment security, such that mothers of infants classified as securely attached are more sensitive and responsive to their infant's signals and communications than mothers of infants classified as insecurely attached (Ainsworth et al., 1978). Moreover, maternal sensitivity during infancy predicts attachment representations well into early adulthood. Beckwith and colleagues (Beckwith, Cohen, & Hamilton, 1999) reported that young adults whose representations of attachment were classified as dismissing had, as infants, been observed to receive less sensitive maternal care than those judged 16 years later to have secure and preoccupied representations. However, among young adults who experienced sensitive care during infancy, those who later had adverse experiences such as parental divorce or death, psychiatric or chronic illness in the family, physical or sexual abuse, showed a higher rate of insecure attachments than their peers whose lives remained benign, suggesting that early sensitive care cannot overide subsequent life events, particularly adverse ones. In addition, parents' representation of their attachment experiences within their family of origin as assessed with the Adult Attachment Interview (George, Kaplan, & Main, 1985) is strongly associated with the nature of their infant's attachment relationship to them as indicated by classifications in the Strange Situation (see van IJzendoorn, 1995, for a meta-analytic review of the literature). Moreover, these associations have been reported concurrently as well as predictively from interviews with parents conducted prenatally (e.g., Fonagy, Steele, & Steele, 1991). Such findings are quite remarkable given the dissimilarity of the measurements involved: one, a narrative analysis focusing on the adult's discourse regarding early attachment experiences; the other, a standardized laboratory paradigm focusing on the infant's overt behaviors such as proximity seeking and contact avoidance.

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According to attachment theory, a parent's representations of relationships influence the quality of interactions with his or her child (Main, Kaplan, & Cassidy, 1985), and thus the link between parental attachment representations and infant attachment is mediated through parent-child interactions. In line with this theorizing, van IJzendoorn's (1995) meta-analysis of 18 studies found a reliable association between a parent's classification on the Adult Attachment Interview and measures of parental responsiveness, as well as evidence for the role of maternal sensitivity as a mediator of the link between adult attachment representations and the mother-infant attachment relationship (see also Pederson, Gleason, Moran, & Bento, 1998). However, as is the case with the link between maternal sensitivity and infant attachment security (De Wolff & van IJzendoorn, 1997), the magnitude of this mediational path is modest, accounting for less than 25% of the relation between adult representation in the AAI and Strange Situation classification. In sum, the construct of maternal sensitivity has been broadened from its initial focus on prompt and appropriate responsiveness to infant signals to include a more systematic analysis of contingency of interaction and to emphasize other aspects of mother-infant interactive behavior, particularly in the domains of affect expression and emotion regulation. Its measurement has varied from microanalytic techniques that attempt to capture the reciprocal nature of early mother-infant interactions to global qualitative rating scales. With and without these theoretical and methodological extensions, maternal sensitivity remains a consistent predictor of attachment security from infancy well into adulthood in normative samples. Moreover, the strength of this predictive link has been demonstrated cross-generationally, as maternal sensitivity appears to mediate the link between parents' own representations of attachment and the nature of their infant's attachment to them. Although in discussing maternal sensitivity we have necessarily focused on the role of maternal behavior in the formation of secure attachment relationships, this behavior cannot be examined independently of the context of child behavior in which it occurs. In the following section, we present a body of research that has attempted to investigate the impact of child characteristics on maternal behavior and infant attachment security in normative samples by focusing on infant temperament. Similarly, we present evidence from research with atypically developing children that documents the impact of altered development on the formation of mother-infant attachment.

IV. Role of Child Characteristics Because the link between infant attachment security and maternal sensitivity is typically modest, factors beyond sensitivity may determine whether an infant develops a secure or an insecure attachment (De Wolff & van IJzendoorn, 1997;

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Goldsmith & Alansky, 1987). A number of theorists have begun to emphasize the need to consider maternal sensitivity as a dynamic construct to be viewed within the context of interactive processes of the dyadic relationship (e.g., Pederson & Moran, 1995; Seifer & Schiller, 1995; Sroufe & Fleeson, 1986, 1988). In addition to considering the role of a mother's sensitivity and responsiveness in shaping infant attachment, researchers have emphasized the need to consider the child's contributions to the attachment relationship (Belsky, 1997; Schneider-Rosen & Rothbaum, 1993; van den Boom, 1997), particularly as these may affect the expression of attachment behaviors in the Strange Situation and the degree to which mothers can act sensitively. A. TEMPERAMENTIN TYPICALLYDEVELOPINGCHILDREN Among typically developing children, temperament, thought to reflect endogenously organized traits that appear early in development and are moderately stable over time, has been widely studied as a possible source for variability in quality of attachment. Temperament is a multiconstruct phenomenon that has been conceptualized within a wide range of theoretical approaches (Goldsmith et al., 1987), each of which has generated different dimensions to the construct. Some researchers have conceptualized temperament as the style in which behavior is exhibited (Chess & Thomas, 1982; see also Thomas & Chess in Goldsmith et al., 1987), independent of the content of the behavior or its motivational context. The expression of this behavioral style, reflecting such dimensions as activity level, adaptability, approach/withdrawal, distractibility, and intensity of affect expression, is sensitive to the constraints of different social contexts and thus codetermines how different social partners respond to a child (Vaughn & Bost, 1999). Other theorists have also focused on the physiological mechanisms underlying reactivity and regulation as potential dimensions of temperament (Goldsmith et al., 1987; Rothbart & Derryberry, 1981). This biobehavioral view encompasses reactivity, mainly the arousal of motor, affective, and autonomic nervous system domains, as well as the capacity for self-regulation, reflecting the child's ability to modulate his own level of reactivity. Negative reactivity/irritability reflects a propensity for distress that may be particularly salient in the child's response to novel stimuli and situations (Vaughn & Bost, 1999). Similarly, Kagan's (1984, 1994) dimension of behavioral inhibition implicates a lowered threshold of excitability/reactivity in children's increased experience of fear and wariness in response to unfamiliar people and events. Cutting across many conceptualizations of infant temperament is an emphasis on the expression and/or experience of affect, particularly negative affect, and the tendency to express this affect in reaction to novelty in the environment, as manifested in infant difficulty, irritability, soothability, and the like (Vaughn & Bost, 1999). Moreover, a certain degree of psychomotor arousal and activity as well as

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the ability to regulate this arousal are central characteristics of many approaches to temperament. Controversy surrounds the role of temperament in the development of attachment. Attachment theorists contend that infant temperament does not exert a main effect in determining the quality of attachment as a sensitive mother can adapt to the temperamental differences in her child (Ainsworth et al., 1978) and that even a difficult child can become secure given the right quality of care (Sroufe, 1985). In contrast, many temperament theorists argue that variation in the security of attachment reflects temperamental differences among infants, particularly their susceptibility to distress. Thus the infant's temperament directly affects the development of attachment, both by affecting behaviors used to classify attachment security in the Strange Situation and by impacting the quality of the mother-infant interaction (Belsky, 1999; Kagan, 1982). A provocative claim advanced by some theorists is that individual differences in patterns of attachment behaviors observed in the Strange Situation (i.e., proximity seeking, contact resistance/avoidance) reflect individual differences in temperament, not the consequences of differences in quality of early mother-child interaction (Kagan, 1984). A child who is more inhibited behaviorally and reactive may appear fearful, restrained, and avoidant in response to the novelty of the Strange Situation itself as well as to the presence of a stranger (Kagan, 1984; Rothbart, 1981). Similarly, differences in reactivity may also influence the degree to which infants become distressed by the separations in the Strange Situation, and an infant's soothability and distractibility will directly influence the relative ease with which the infant returns to baseline levels of arousal when distressed (Belsky & Rovine, 1987; Seifer & Schiller, 1995). Moreover, less sociable infants may also be more likely to show avoidant behavior upon being reunited with the mother. Finally, the child's activity level will influence its tendency to explore the environment in the Strange Situation and return to the attachment figure and thus carries direct implications for assessment of secure-base and haven-of-safety phenomena (Seifer & Schiller, 1995). In sum, temperamental differences may directly increase the likelihood of insecurity through their effects on the expression of behaviors used to classify attachment security in the Strange Situation. In fact, however, in typically developing children, temperament is only modestly related to security/insecurity differences in the Strange Situation, thus questioning claims that variations in attachment security are a direct product of temperamental differences (Goldsmith & Alansky, 1987; Vaughn & Bost, 1999). Nonetheless, infant temperament may still indirectly affect the development of attachment via its impact on the nature and quality of mother-infant interactions (Chess & Thomas, 1982; Kagan, 1982). Central to this proposal is the claim that temperamentally difficult childrenmthose who experience more intense negative affect and who are perceived as more difficult and more prone to distress--may be more likely to elicit less than optimal caregiving (Vaughn & Bost, 1999). Similarly, more reactive,

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irritable, and distractible infants may be less likely to experience synchronous interactions with their mothers, and thus they "miss-out" on opportunities for successful mutual regulation (Seifer & Schiller, 1995). Evidence for this indirect model comes from several studies examining the influence of infant temperament on quality of mother-infant interaction. Seifer and colleagues (Seifer, Schiller, Sameroff, Resnick, & Riordan, 1996) reported that observers' ratings of maternal sensitivity were associated with observers' assessments of infants' mood and approach behaviors although no associations were found with mothers' perceptions of infant temperament. Thus, sensitive mothers have infants who are judged by observers--but not by their mothers--to be less difficult temperamentally. Van den Boom (1994) in her longitudinal investigation of irritable infants and their mothers, designed an intervention intended to improve the mothers' sensitivity to infant communicative signals, and she found that insecurity was common in the control group but reduced significantly in the intervention group, approaching that reported for comparable groups unselected for irritability (van den Boom, 1994). Thus, as postulated by theorists of temperament, infants' temperamental quality of irritability did increase rate of insecurity in the control group; but, as proposed by attachment theorists, maternal sensitivity, increased by intervention, directly related to an increased occurrence of security, even in infants who were initially very irritable. Moreover, Crockenberg (1981) reported an interaction between infant irritability and mothers' social support, such that infants' irritability was not significantly associated with insecure attachments when mothers received high levels of social support. These results suggest that while maternal sensitivity may be affected by infants' temperament, infants' temperament does not determine the quality of the attachment relationship. In summary, two general approaches to the contribution of infant temperament to the attachment relationship have been evaluated empirically. Although infant temperament does not appear to directly affect attachment by influencing the expression of behaviors used to classify attachment security in the Strange Situation, empirical evidence supports the claim that it does so indirectly, by affecting the quality of the mother-infant interactions that foster the development of a secure attachment. But the influence is modest and overcome by many parents. Findings that variations in infant temperament influence mothers' ability to respond to their infants' signals promptly and appropriately indicate that the development of attachment can not be considered as a one-sided process, primarily driven by the consistency and quality of maternal behavior. Rather, it must be viewed as a dynamic process that develops in the context of an interaction between child characteristics and maternal behavior. This contextual approach to the development of attachment may be particularly fruitful in the study of atypically developing children, where alterations in children's cognitive, socioemotional, and physical abilities represent a wider range of possible sources of variability in child characteristics that have the power to affect

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maternal behavior and thus ultimately the development of attachment. Equally important is a consideration of the extent to which maternal behavior relevant to the development of infant attachment is altered among atypical mothers, those who abuse drugs and alcohol as well as those with various types of psychopathology. In the following section, we present evidence of the impact of alterations in both child and maternal characteristics on the development of attachment. B. ATYPICALLY DEVELOPING CHILDREN

Just as literature charting the cognitive and social development of typically developing children has informed our understanding of how the achievement of major developmental tasks is altered in atypical groups of children, so research with atypically developing children has the power to illuminate the processes underlying typical developmental trajectories. By studying how disruptions in the developmental process affect attachment in children who suffer from developmental delays, sensory deficits, and physical handicaps, we begin to expose the contribution of cognitive and socioemotional abilities to the development of attachment in all children. Because a great deal of homogeneity is expected with respect to the level of social and cognitive skill that is achieved by typically developing children at various stages of development, there has been very little consideration in the attachment literature of the extent to which these abilities contribute to the development of attachment. By studying the development of attachment in children who vary more greatly in their cognitive, linguistic, and socioemotional abilities, we begin to understand to what extent these abilities may in fact represent foundations on which attachment is developed. A number of researchers have in fact begun to consider the contribution of children's characteristics to the development of attachment security in atypical groups of children, particularly their impact on maternal sensitivity. Mothers can respond promptly and appropriately only when children communicate their needs effectively. That is, mothers' responding is probably influenced by a number of features of infants' behavior, such as clarity of facial/affective expressions, integration of different communicative modes (facial, vocal, and gestural), and intensity and timing of communicative signals. If children in nonnormative groups (e.g., children with developmental delays, physical handicaps, and chronic medical conditions) are unable clearly to communicate their needs, this would tend to interfere with and limit the degree to which mothers can be sensitive to their children's signals. A growing body of research on maternal-child interaction reveals that mothers of atypical children are often less sensitive. Mothers of hearing-impaired infants (Henggeler, Watson, & Cooper, 1987), infants with physical handicaps (Wasserman, Allen, & Solomon, 1985) and cerebral palsy (Kogan & Tyler, 1973), preterm infants (Brachfeld, Goldberg, & Sloman, 1980; Brown & Bakeman, 1980; Crnic, Greenberg, Ragozin, Robinson, & Basham, 1983; Crnic, Ragozin,

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Greenberg, Robinson, & Basham, 1983; Field, 1977a, 1977b; Goldberg, Brachfeld, & DiVitto, 1980), and infants with Down syndrome (Jones, 1977; Marfo, 1990; Tannock, 1988) have been described as more directive and dominant and less responsive than mothers of typically developing children. These alterations in patterns of mother-infant interaction have been attributed to disabled children's tendency to display more unreadable cues (Goldberg, 1977; Hauser-Cram, Warfield, Shonkoff, & Krauss, 2001; Hyche, Bakeman, & Adamson, 1992), and delays and deficits in behaviors used to elicit maternal interaction (Cicchetti & Pogge-Hesse, 1982). For example, Moran, Pederson, Pettit, and Krupka (1992) reported that mothers in their sample of developmentally delayed infants (diagnosed as having genetic disorders, premature birth, neurological impairments, and significant developmental delays) were less sensitive than mothers of nondelayed children, and they attributed this difference to "developmentally delayed infants' relative inability to provide the stimuli and responses necessary to support sensitive interaction with their caregivers" (p. 439). Although this body of research suggests that atypically developing children do in fact affect their mothers' capacity to parent them with sensitivity, few investigators have systematically examined the impact of alterations in children's social and cognitive abilities on the quality of attachment in atypical groups. Mothers may be rated as more dominant and intrusive in their interactions with their delayed infants, but this does not necessarily imply that these infants will develop higher rates of insecure attachment if this maternal behavior is adaptive for the delayed infant, or if the attachment formation process itself proceeds differently in these groups. Alternatively, in line with research on typically developing children, decreased maternal sensitivity may disrupt the formation of mother-infant attachment in atypical groups, resulting in higher rates of insecure attachment. In this section we take the first step of choosing among these alternatives by examining the extent to which the literature documenting alterations in children's characteristics and maternal behavior within several atypical groups is reconciled with reported rates of attachment security. 1. Preterm Infants

Infants born prematurely have generally been described as more easily aroused and irritable (Brachfeld et al., 1980; Brown & Bakeman, 1980; Crawford, 1982; Field, 1977a, 1982; Frodi et al., 1978) and "less able to maintain states of alertness without becoming disrupted into drowsiness or fussing and cries" (Beckwith & Cohen, 1989, p. 76) and thus more difficult to soothe and comfort (Brown & Bakeman, 1980; Field, 1977a; Friedman, Jacobs, & Werthmann, 1982; Frodi & Thompson, 1985). Differences between term and preterm infants have also been noted in terms of alertness and responsiveness to stimulation (Anders & Keener, 1985; Barnard, Bee, & Hammond, 1984; Brachfeld et al., 1980; Crnic, Greenberg, et al., 1983; Crnic, Ragozin, et al., 1983; Field, 1982; Frodi & Thompson, 1985;

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McGehee & Eckerman, 1983), with preterm babies experiencing a higher threshold for orienting and attentive responses (Field, 1982). This low tolerance for stimulation and difficulty with modulating arousal, combined with a higher threshold for responding to social stimulation, may pose a particular challenge to maintaining smooth interactions, and it may result in less synchronous exchanges between mothers and their preterm infants (Goldberg, 1977, 1978). The lack of clarity and consistency in their infant's behavior (Goldberg, 1979), as well as decreased social responsiveness, may force mothers to be more active and controlling in engaging with their preterm infants (Barnard et al., 1984; Brachfeld et al., 1980; Crnic, Greenberg, et al., 1983; Crnic, Ragozin, et al., 1983; Field, 1977a) and thus to be less contingent on their infant's cues (DiVitto & Goldberg, 1983; Goldberg & DiVitto, 1995). Moreover, maternal sensitivity in this population may be specifically affected by the degree of infants' impairment. For example, mothers of preterm infants who have higher IQs and who make more nondistress vocalizations in the early months of life have been rated as more responsive than mothers of lower functioning and more irritable infants (Beckwith & Cohen, 1989; Goldberg, Lojkasek, Gartner, & Corter, 1989). Jarvis, Myers, and Creasey (1989) also demonstrated that severity of complications following preterm birth contributes to observed differences in interactional patterns. Mothers of infants who had experienced more severe respiratory problems in the early months of life were rated as less sensitive and responsive to infant distress than mothers of healthier infants (see also Minde, Whitelaw, Brown, & Fitzhardinge, 1983). Despite these widely documented differences in interactional patterns between mothers and their preterm infants, rates of security among preterm infants are the same as for full-term comparison groups (Easterbrooks, 1989; Field, Dempsey, & Shuman, 1981; Frodi, 1983; Frodi & Thompson, 1985; Goldberg, 1988; Goldberg, Perrotta, Minde, & Corter, 1986; Rode, Chang, Fisch, & Sroufe, 1981; Rodning, Beckwith, & Howard, 1989; although see Plunkett, Meisels, Stiefel, Pasick, & Roloff, 1986). As Goldberg and DiVitto (1995) concluded in their review of the literature on parenting preterm children, in spite of observed differences in patterns of mother-infant interaction the "bulk of the data indicates that by the end of the first 12 to 18 months of life, preterm infants and their parents have developed a relationship that, with respect to attachment security, does not differ from that of term infants and their parents" (p. 223). Notable, however, is an increase in marginally secure subclassifications that have been reported in a few studies and the related finding that mothers of marginally secure infants appear less sensitive than mothers of insecurely attached infants (Goldberg, 1988; Goldberg et al., 1986). This anomalous finding may imply different precursors of security and insecurity in preterm groups or may result from forced classifications of infants into marginally secure classifications where disorganized or unclassifiable categories may have been more appropriate. In

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a meta-analysis of studies in which the disorganized/disoriented classification was used, 24% of preterm infants were in this category, compared to the 15% found among typically developing comparison groups (van IJzendoorn, Goldberg, Kroonenberg, & Frenkel, 1992). Moreover, when preterm birth is combined with serious continuing medical conditions, (Plunkett et al., 1986), or a low socioeconomic family environment (Wille, 1991), secure attachment is less frequent. Similarly, Poehlmann and Fiese (2001), found that preterrn birth interacted with maternal vulnerability, so that preterms being reared by mothers with depressive symptoms were most likely to be insecure, whereas maternal depressive symptoms were unrelated to quality of attachment for the full-term infants. How can these seemingly paradoxical findings--decreased maternal responsiveness and increased intrusiveness, normative percentages of attachment security, and increased occurrence of disorganized attachment--be reconciled? Researchers have suggested that observed differences in maternal interacfional style, rather than reflecting overstimulation and intrusiveness, may in fact be compensatory and adaptive for the preterm infant (DiVitto & Goldberg, 1983; Field, 1977b, Goldberg, 1988; Goldberg & DiVitto, 1995). Increased maternal activeness and directiveness/control may reflect a good strategy for maintaining and supporting interactions with an infant who is less responsive and attentive and incapable of self-regulation. Findings of attachment studies thus indicate that most mothers are in fact responding to their preterm infants in ways that are appropriate and promote their development (Goldberg & DiVitto, 1995). But the increased occurrence of the atypical disorganized attachment pattern, albeit in a minority of preterm infants, points to the increased vulnerability of the infant to alterations in development. 2. Children with Sensory Deficits Interactions between mothers and children who are blind (Werth, 1984) or deaf (Lederberg & Mobley, 1991) are strongly affected by the disruption of normal means of mother-infant communication, particularly disruptions in the synchrony of modalities used for mutual cueing (Werth, 1984). Blind and deaf children may thus appear unresponsive to their mothers, and conversely, mothers may appear as more intrusive and insensitive. Observational research on interactions between deaf children and their mothers indeed indicates that deaf children are less responsive and compliant and less likely to initiate interactions with their mothers (Henggeler & Cooper, 1983; Meadow, Greenberg, Erting, & Carmichael, 1981; Schlesinger & Meadow, 1972; Spencer & Gutfreund, 1990; Wedell-Monnig & Lumley, 1980), and their mothers tend to be rated as more controlling and dominant and less positive (Brinich, 1980; Henggeler & Cooper, 1983; Henggeler et al., 1987; Nienhuys, Horsborough, & Cross, 1985; Schlesinger & Meadow, 1972; Spencer & Gutfreund, 1990; Wedell-Monnig & Lumley, 1980). Nonetheless, some researchers report that hearing-impaired mother-infant dyads appear as responsive to one another

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(Spencer & Gutfreund, 1990) and as synchronous in early face-to-face interactions (Nienhuys et al., 1985) as hearing dyads. Although a few investigators have reported that deaf children display the same attachment behaviors, such as separation distress, as heating children (Greenberg & Marvin, 1979; Meadow, Greenberg, & Erting, 1984), only Lederberg and Mobley (1991) actually observed deaf toddlers and their mothers in the Strange Situation. Hearing-impaired and heating toddlers did not differ in the frequency of secure, insecure, and disorganized attachments. Moreover, despite the fact that deaf dyads spent less time interacting during free play, had more maternal initiations, and miscommunicated more frequently, hearing impairment did not affect quality of mother-toddler interaction. Mothers of hearing impaired toddlers did not differ on qualitative ratings of dominance and directiveness, but rather they were rated as equally sensitive and affectively positive as mothers of heating toddlers. Thus, despite communicative barriers, hearing-impaired infants and their mothers appear as likely to establish positive, reciprocal, and secure relationships as heating dyads. 3. Children Who Are Chronically Ill or Physically Handicapped

Children suffering from chronic medical conditions, such as cystic fibrosis and cerebral palsy, as well as physically handicapped children, may place a quite different strain on the mother-child relationship. The stress and anxiety associated with care of children who may require constant supervision and assistance (Bell, 1981; Goldberg, Morris, Simmons, Fowler, & Levison, 1990; Hobbs, Perrin, & Ireys, 1985; Kornblatt & Heinrich, 1985; Shapiro, 1983)and feelings of mourning and unresolved loss regarding the birth of a chronically ill or handicapped child (Marvin & Pianta, 1996) may have a tremendous impact on the quality of motherinfant interactions in these groups. This may be particularly true in the case of cystic fibrosis, due to the fear and anxiety associated with daily treatment and feedings, as well as the child's failure to thrive and progressive deterioration (e.g., Simmons, Goldberg, Washington, Fischer-Fay, & Maclusky, 1995). Despite these potential stressors on the mother-infant relationship, the distribution of attachment patterns in physically handicapped infants (Wasserman, Lennon, Allen, & Shilansky, 1987) does not differ from normative data, suggesting a less pervasive impact of child characteristics on quality of mother-infant interaction. Wasserman and colleagues (1987) reported that although mothers of physically handicapped infants were less responsive than mothers of the typically developing control infants, they were more available and affectively positive. Because groups did not differ in attachment organization, Wasserman et al. (1987) interpret these findings to mean that a highly active interactive style (positive affective involvement combined with lowered responsivity) may in fact be an adaptive strategy promoting the development of secure attachment in this group. These findings accord well with those reported for premature infants, in emphasizing that although children's characteristics may alter maternal interaction styles, the resulting

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interactive patterns may still be adaptive for the handicapped child (Goldberg, 1988; Goldberg & DiVitto, 1995). In contrast, insecure attachment is more common among children with cerebral palsy (54%; Marvin & Pianta, 1996; Stahlecker & Cohen, 1985) and children with medical conditions that are unpredictable and potentiate continuing crises, such as cystic fibrosis and congenital heart defects (57%; Goldberg, Gotowiec, & Simmons, 1995). Notably, Marvin and Pianta (1996) found that the severity of the child's disability (e.g., whether they could or could not locomote) did not affect security of attachment, but maternal reactions to their children's diagnosed disabilities did. Thus, children's characteristics did not directly affect quality of attachment, but did so indirectly through affecting maternal feelings. Whereas mothers can accommodate to their children's physical deficits, they are less able to adapt when their children's conditions arouse continuing anxiety and strain their ability to come to terms with their child. Some researchers have suggested that the amount of social support may buffer the effects of having a handicapped or chronically ill child on the mother-infant relationship (Capuzzi, 1989; Lojkasek, Goldberg, Marcovitch, & MacGregor, 1990). 4. Children with Down Syndrome

Children with Down syndrome have been characterized as more neutral and less emotionally expressive, particularly in response to social stimuli, and their affective displays have been described as briefer and less intense than those of typically developing children (Bridges & Cicchetti, 1982; Brooks-Gunn & Lewis, 1982; Cicchetti & Serafica, 1981; Cicchetti & Sroufe, 1978; Emde, Katz, & Thorpe, 1978; Ganiban, Barnett, & Cicchetti, 2000; Kasari & Sigman, 1996; Sorce & Emde, 1982; Thompson, Cicchetti, Lamb, & Malkin, 1985). Such muted affect and ambiguous affective cues may strain mothers' ability to perceive and interpret their children's signals, and thus limit their ability to respond consistently and sensitively to their communications (Bridges & Cicchetti, 1982; Kasari & Sigman, 1996). Disruptions in emotional signaling may also increase mismatches in parental responding (Brooks-Gunn & Lewis, 1982; Kasari & Sigman, 1996). Mothers may react to decreased expressivity by becoming directive and controlling in their interactions with their infants (Field et al., 1980; Tannock, 1988), resulting in fewer opportunities for children with Down syndrome to interact synchronously with their mothers. Despite the large body of research on altered emotional signaling and maternal interactive behaviors in Down syndrome, few investigators have addressed these patterns in the context of the attachment relationship. The existing studies find that in children with Down syndrome, secure attachment is less common (e.g., 40% secure at 26 months and 48% at 42 months) and disorganized/disoriented or unclassifiable attachments are more common (e.g., 49% at 26 months and 48% at 42 months; Atkinson et al., 1999; see also Ganiban et al., 2000; van IJzendoorn

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et al., 1992; Vaughn et al., 1994). Vaughn and colleagues (1994) attributed the large proportion of disorganized and unclassifiable cases to the methodological issue that children with Down syndrome may not be sufficiently stressed by the Strange Situation. Yet, Ganiban et al. (2000) found no consistent relationships between negative reactivity and attachment behaviors and organization among children with Down syndrome, suggesting that an alternative explanation is needed. That is, if children with Down syndrome who were and were not distressed by the Strange Situation both showed disorganized attachment organization, then the fact that children with Down syndrome tend to show less stress in the situation than typically developing children cannot be the crucial factor in the elevated rates of disorganization in this population. Evidence on the validity of attachment classification among children with Down syndrome comes from work by Atkinson et al. (1999), who specifically addressed the influence of child functioning and maternal sensitivity as antecedents of attachment security in this group of children. Whereas mothers of securely attached infants were rated as more sensitive than mothers of insecure (avoidant, resistant, disorganized) and unclassifiable children, more cognitively competent infants were also more likely to be classified as securely attached than lower functioning infants. Moreover, a combination of relatively high cognitive skill and maternal sensitivity best predicted secure attachments among this group. These findings support the link between maternal sensitivity and attachment security among children with Down syndrome, indicating that disorganized or unclassifiable attachment classifications in this population reflect maternal behavior in the home rather than artifacts of the Strange Situation. They also bear on the debate regarding the impact of children's characteristics on maternal sensitivity. The interaction among sensitivity and cognition suggests that the level of developmental functioning among children with Down syndrome may constrain maternal sensitivity: Affective signaling may be particularly disrupted among children with Down syndrome with particularly limited cognitive functioning, such that these children elicit less sensitive caregiving and, consequently, are at greater risk for developing insecure attachments. 5. Children with Autism

As autism has been characterized by a failure to develop affective contact with other people (Kanner, 1943) and by deficits in the ability to share others' internal experiences, such as their intentions, feelings, and thoughts (Hobson, 1991), this developmental disorder may be fertile ground for investigation into the impact of child characteristics on maternal sensitivity. Given autistic children's severe socioemotional and communicative impairments (American Psychiatric Association, 1994), their signals and communications may be very idiosyncratic, subtle, and ambiguous, and thus difficult for the mother to interpret and respond to sensitively. In light of limited language and nonverbal communication, particularly the relative absence of gestures used for sharing and regulating attention (Loveland &

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Landry, 1986; Mundy, Sigman, & Kasari, 1994; Mundy, Sigman, Ungerer, & Sherman, 1986; Sigman & Ruskin, 1999; Sigman, Mundy, Sherman, & Ungerer, 1986), children with autism may be less likely to communicate their needs and intentions in the first place. Moreover, when they do signal, children with autism are more likely to display incongruous and negative blends of facial expression of emotion during social interactions (Loveland et al., 1994; Kasari, Sigman, Yirmiya, & Mundy, 1993; Yirmiya, Kasari, Sigman, & Mundy, 1989), and they rarely coordinate their facial expressions of emotion with other aspects of communication, such as gaze (Dawson, Hill, Spencer, Galpert, & Watson, 1990; Joseph & TagerFlusberg, 1997; Snow, Hertzig, & Shapiro, 1987) and joint attention behaviors (Kasari, Sigman, Mundy, & Yirmiya, 1990). This lack of coherence in emotional signaling, combined with a decreased tendency to use affect in a coordinated and communicative way, may severely constrain caregivers' ability to read and interpret their children's signals (Kasari & Sigman, 1996; Kasari et al., 1990; Kasari, Sigman, Yirmiya, et al., 1993) and thus to consistently respond to these signals in a sensitive manner. Alterations in children's social responsiveness may be just as crucial in disrupting the establishment of harmonious mother--child interactions as deficits in signaling. Children with autism appear less attentive to social stimuli (Dawson, Meltzoff, Osterling, & Rinaldi, 1998; Dawson, Meltzoff, Osterling, Rinaldi, & Brown, 1998; Hermelin & O'Connor, 1970; Kasari, Sigman, Yirmiya, et al., 1993; Osterling & Dawson, 1994; Sigman & Capps, 1997) and less responsive to emotions and initiations of other people (Bacon, Fein, Morris, Waterhouse, & Allen, 1998; Kasari, Sigman, Baumgartner, & Stipek, 1993; Sigman & Ruskin, 1999; Sigman, Kasari, Kwon, & Yirmiya, 1992), and thus their mothers may encounter greater difficulties in affect attunement and in establishing contingent and synchronous interactions with their children (e.g., Dawson et al., 1990; Gartner & Schultz, 1990). Early clinical observations of autistic children's failure to respond to separations and reunions were interpreted to mean that autistic children do not develop differential attachments to their caregivers (Kanner, 1943; Rutter, 1978). In contrast, research findings challenge the belief that these children fail to develop attachments (Shapiro, Sherman, Calamari, & Koch, 1987). Capps and her associates (1994) found that all the children in their sample were given a primary classification of "disorganized." However, when the criteria for assignment of the disorganized classification were revised so that autistic stereotypies such as handflapping, toe-walking, and odd noises were not considered, 40% of the children were subclassified as securely attached. Similarly, when Rozga (2001) excluded autistic stereotypies from consideration, 45% of the autistic children in her study were classified as secure. We know relatively little about the interactional antecedents of attachment security in children with autism. Capps et al. (1994) reported that although mothers of securely attached children with autism were significantly more sensitive than

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mothers of insecurely attached children, securely attached infants had advanced receptive language abilities and were more likely to make requests of and initiate social interactions with their mothers. These relationships were confirmed in a short-term longitudinal study (Rozga, 2001), reporting that although mothers of securely attached children displayed greater sensitivity, the securely attached children had higher mental ages, receptive and expressive language abilities, and nonverbal communication skills, concurrently as well as during a 1-year follow-up. Moreover, maternal sensitivity was correlated significantly with children's nonverbal communication skills and expressive and receptive language abilities, such that mothers of higher functioning children were rated as more sensitive than mothers of lower functioning children. Attachment security appears to be strongly associated with autistic children's language and cognitive skills (Capps et al., 1994; Rogers, Ozonoff, & MaslinCole, 1991, 1993; Rozga, 2001; Willemsen-Swinkels, Bakermans-Kranenburg, Buitelaar, van IJzendoorn, & van Engeland, 2000) as well as social-interactive behaviors (Rozga, 2001). These associations among attachment security, children's functioning, and maternal sensitivity in autism echo the findings of Atkinson and colleagues (1999) with Down syndrome children, suggesting that among atypically developing children, their level of cognitive functioning may affect their mothers' ability to perceive and read their signals, and thus alter their ability to parent them with sensitivity. Attachment theory does not posit associations between individual differences in cognitive skill and quality of attachment, yet these links are found among children with autism and with Down syndrome, suggesting that infants' characteristics play a greater role in maternal sensitivity and the development of attachment in atypically developing children than previously considered in typically developing children. As this review of attachment in atypical groups reveals, despite the wide range of physical, cognitive, and socioemotional deficits characterizing children with developmental disabilities, the process of attachment formation is not disrupted uniformly in these groups. Rates of attachment security among infants with hearing impairments and physical handicaps typically do not differ from those reported for typically developing infants, suggesting that despite disruptions in normal means of mother-child communication and increased stress on the part of the parent, these infants and mothers adapt their behavior in ways that promote the formation of secure attachment. However, when children's delays extend to cognitive impairments and deficits in social responsiveness and emotional expressiveness, as is the case with children with autism, Down syndrome, and, to a lesser degree, infants born preterm, rates of insecurity, particularly attachment disorganization, are greatly increased. These findings suggest that, in line with our contextual approach to attachment formation, yet unspecified levels of cognitive and social skill on the part of the child may be prerequisite for the development of secure mother-infant attachment. Our

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analysis of how child characteristics interact with maternal sensitivity in shaping the quality of the attachment relationship cannot be complete, however, without a consideration of variations in maternal characteristics relevant to attachment. By examining the literature on mothers who abuse drugs and alcohol or who suffer from various forms of psychopathology, such as depression, we begin to unravel the extent to which maternal sensitivity and infant attachment are also affected by the mothers' own characteristics.

V. Atypical Groups of Mothers Atypical groups of mothers, including those who abuse alcohol and drugs and/or are marked by psychopathology, are less likely to be sensitive to their child, and their child's attachment relationship to them is more likely to be atypical (van IJzendoom et al., 1992). The number of prospective, longitudinal studies of caregiving in atypical mothers, however, is few (Schneider-Rosen & Rothbaum, 1993) and for some groups little empirical information is available about the caregiving environments provided to the infants. A. MOTHERS WHO ABUSE DRUGS AND ALCOHOL

Beckwith and colleagues have conducted two minilongitudinal studies in the inner city of Los Angeles with mothers of low educational attainment and minority status who were living in poverty (Espinosa, Beckwith, Howard, Tyler, & Swanson, 2001; Rodning, Beckwith, & Howard, 1991; Swanson, Beckwith, & Howard, 2000). One was a study of the offspring of women who used PCP and cocaine during pregnancy, as compared to nondrug using mothers living in the same residential area. The children prenatally exposed to drugs grew up in a variety of caregiving contexts: some lived with their biological mothers, some with kinship caregivers, and some in professional foster homes. In contrast to the comparison group, they were unlikely to live with their biological fathers, and they were likely to have many caregivers, to have higher rates of foster care placement, and to have experienced unpredictable changes from one primary caregiver and living arrangement to another (Rodning et al., 1991). Systematic observations done during unstructured home visits, when the infants were 3 and 9 months of age, indicated that the biological, substance-abusing mothers were less responsive, more rejecting, more ignoring and neglecting, more interfering, and had poorer quality of physical contact than the non-substanceabusing comparison mothers. The insensitivity apparently was not determined by drug-related changes in children: For the small number of infants born to substanceabusing mothers who were placed in professional foster care, their foster parents earned high sensitivity scores.

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When the infants were 15 months old, they were tested in the Strange Situation. The rate of disorganized attachment was very high, 68%, resulting in a severe reduction in security, 32%, in comparison to the offspring of families living in poverty in the same neighborhoods of whom the majority, 64%, were securely attached. Those infants who were classified as secure across both groups were significantly more likely to have had caregivers who were more sensitive and more cooperative with the baby, although the associations were only modest. A perplexing finding was that the high levels of sensitivity experienced by the drugexposed infants with professional foster mothers did not increase their likelihood of forming a secure attachment relationship. The reason was unclear, but supports the conclusion that factors yet to be identified, in addition to parental sensitivity, contribute to child security. Also surprising was that change in primary caregiver, or the number of such changes (e.g., from mother to grandmother, or foster mother to grandmother to mother), was not related to insecurity. The study described only the first 2 years of the children's lives, and the long-term implications for development are not known (Rodning et al., 1991). The second study included low education, minority status, poverty-dwelling mothers who were chronic, heavy users of cocaine and other drugs. The study began in pregnancy and continued to 2 years after birth (Beckwith, Howard, Espinosa, & Tyler, 1999; Espinosa et al., 2001; Swanson et al., 2000). Maternal psychopathological symptoms were assessed during the prenatal period via a systematic, well-standardized self-report questionnaire. Depressive symptoms were assessed again when the infants were 6 and 18 months of age. Caregiver sensitivity was observed in the home and laboratory when the infants were 1 and 6 months of age. At 18 months of age, toddlers' attachment to their caregivers was assessed through the Strange Situation procedure. Attachment classifications reflected a high rate of disorganization, 45%, and a low rate of security, 37%. Biological mothers and kinship caregivers did not differ on their toddlers' attachment security or their sensitivity, intrusiveness, and hostility to the child. There were too few children living with nonkinship caregivers to compare statistically. However, it was noteworthy that each of the 3 children whose primary caregiver was nonkin was assessed as secure. Toddlers who had remained continuously with their biological mother from birth did not differ in distribution of attachment classifications when compared to those toddlers who had experienced changes in primary caregiver. Although separation from the mother theoretically is a primary experience contributing to the development of an insecure infant-mother attachment (Bowlby, 1973), its effects may be masked by the insecurity resulting from a consistent relationship with an insensitive mother. Among those infants growing up with their biological mothers, symptoms of maternal psychopathology during pregnancy were found to be associated with both the style of mother-infant interaction and the organization of the child's attachment to the mother, but the findings were both complex and surprising. Severe

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psychological symptoms within the clinical range, especially symptoms of depression and paranoia, were very common during pregnancy among the mothers in this group, with only 14% of them not experiencing a level of depressive or paranoid symptoms within the clinical range. Mothers demonstrating clinical levels of paranoia during pregnancyJalmost 55% of the substance-abusing group--later provided less sensitive caregiving and had toddlers who were more likely to be disorganized/disoriented in their attachments. Surprisingly, those mothers with clinical levels of prenatal dysthymia that did not continue during infancy and that was not combined with paranoid symptoms were more sensitive in their early caregiving and had toddlers who were significantly more likely to be secure. Although the latter findings appear startling, they are consistent with the course and sequelae of depressive symptoms from the prenatal to the postpartum period among non-drug-using women. Campbell, Cohn, Flanagan, Popper, and Meyers (1992) found that among women who were delivering their first child, and who were recrutied because of diagnosed depression, most women's depressions had lifted by the time their infants were 6 months of age, and for those women, both their caregiving and the infants' development were not affected. Therefore, recognition of the increased likelihood of severe depressive symptoms among substance-abusing women, particularly in pregnancy (Luthar, Cushing, & McMahon, 1997) while important and accurate, is too narrow a characterization of their psychological problems. Psychopathology in these women is complex, and depression tends not to be a singular problem. When the toddlers were 18 months of age, caregivers of toddlers with avoidant or disorganized/disoriented attachments were found to be more intrusive with their toddlers than caregivers of toddlers with secure attachments (Swanson et al., 2000). Unexpectedly, caregiver sensitivity was not associated with attachment classification, although sensitivity was significantly inversely correlated with intrusiveness. Intrusiveness, rather than sensitivity may have become salient, since the nature and duration of the behavior sample of the study (i.e., brief, structured play interactions in the laboratory) may not have afforded sufficient opportunity for caregivers differentially to display responsiveness to need, a core component of sensitivity. The adverse influence of psychopathology in the parenting of drug-dependent women was supported by Hans, Bernstein, and Henson (1999) in a study of women who abused opioid drugs. Maternal drug use was related to unresponsive and negative parenting. The association was accounted for chiefly by effects of comorbid maternal psychopathology, particularly antisocial, borderline, narcissistic, and histrionic personality disorders. In contrast to the studies we have just described, in which maternal psychopathology was the mediating variable that linked maternal drug use to maternal insensitivity and infants' insecure attachment, O'Connor and her colleagues (O'Connor, 2001; O'Connor, Sigman, & Brill, 1987) found that infant temperament was a significant mediator linking maternal alcohol abuse to infants' insecure attachment. In

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a sample of middle-class women who drank heavily during pregnancy, 48% of their offspring were judged to have disorganized attachments, and only 35% of the offspring were classified as secure. Women who drank more heavily during pregnancy had infants with higher levels of negative affect at 1 year of age. Infants who exhibited higher levels of negative affect were more likely to be insecure in attachment. B. MATERNALDEPRESSION A rich research literature indicates that maternal depression generally is associated with atypical patterns of interaction with the infant and changes in the infants' development. In general, maternal depression is related to higher rates of insecure attachment (Cicchetti, Rogosch, & Toth, 1998; Lyons-Ruth, Connell, Grunebaum, & Botein, 1990; Murray, 1992; Radke-Yarrow, Cummings, Kuczynski, & Chapman, 1985; Seifer, Sameroff, et al., 1996; Teti, Gelfand, Messinger, & Isabella, 1995), but not in all studies (Campbell, Cohn, & Meyers, 1995; DeMulder & Radke-Yarrow, 1991). Research varies widely as to the nature of the depressed samples (clinical samples of women with a diagnosed disorder, or women who sought treatment, or community samples that contain multiproblem women who report high levels of distress on symptom checklists). The studies also differ about the timing and chronicity of maternal depression in the life of the infant and the number of co-occurring adverse factors existing in the family (e.g., single parent, marital discord, comorbid diagnoses, low socioeconomic status) (Cummings & Davies, 1994). Considering those differences provides partial explanations of the inconsistencies found and the substantial variance in effect size among the studies included in meta-analyses (Atkinson, Paglia, Coolbear, & Niccols, 2000; Martins & Gaffan, 2000). For example, the offspring of mothers, mainly middle-class, with diagnosed bipolar depression have been found to show very high (67%) rates of insecurity (DeMulder & Radke-Yarrow, 1991). Teti et al. (1995) studying women with a diagnosed disorder of unipolar major depression who were seeking treatment found only 20% of their infants to show secure attachment in contrast to 70% of the comparison infants. Seifer, Sameroff, et al. (1996) also found significantly increased rates of insecure attachment (64%) in the offspring of women with diagnoses of major depression, regardless of the severity of psychological distress or the number of contextual risks in the family. Cicchetti et al. (1998) found 44% of toddlers of mothers with unipolar depression to be insecurely attached as compared to only 18% in the comparison group. In contrast, Campbell et al. (1995) found in a relatively low-risk community sample of first-time mothers that a diagnosis of depression in the postpartum period was not associated with maternal insensitivity nor attachment insecurity when the depression was short-lived. But when the depression lasted through the first 6 months of the infants' lives, interactions were less positive and insecurity increased but not to the level shown in the Teti et al.

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(1995) study. Another study found that a diagnosed disorder of maternal depression was linked to increased rate of insecurity in the offspring but only for mothers with comorbid diagnoses (Carter, Garrity-Rokous, Chazan-Cohen, & Little, 2001). Children whose mothers were depressed only did not differ in rate of security from children of mothers without psychopathology. The nature of the sample, the timing and chronicity of the depression, and the co-occurring adversities within the family also influence the rate of disorganized attachments among the offspring of depressed mothers. One meta-analysis (van IJzendoorn et al., 1999) found that the occurrence of disorganized/disoriented patterns among offspring of depressed mothers (21%) is only slightly and insignificantly increased over normative groups (15% for comparison, middle-class dyads). The selection and constitution of the depressed sample, however, markedly affected the rate of disorganization that was revealed. Among clinical samples, the rate was significantly increased over normative groups, whereas community samples overlapped with their comparison groups. Disorganized attachments were associated with more severe and more chronic maternal depression. Studies of depression symptom severity, even among women who are undiagnosed, indicate that mothers with severe dysphoric symptoms tend to show flatter affect and to be less contingently responsive, more disengaged, and/or hostile in interaction with their infants (Cohn & Campbell, 1992), particularly as the maternal depressive symptoms become chronic. Their infants also show less attentiveness and are fussier than infants of nondysphofic mothers (Field, 1992). Field (1992) found that if mothers stayed depressed during the first 6 months of their infant's life, their baby appeared depressed as well, and did more poorly at 12 months on the Bayley Scales. However, if the mothers' depressive symptoms lifted by 6 months postpartum, then adverse effects did not occur. As was found with drug-abusing women, comorbid personality disorders (paranoid, schizoid, schizotypal) in women with depression potentiated less engaged and less sensitive interactions with the infant and higher rates of insecurity in the child (Carter et al., 2001; DeMulder, Tarulla, Klimes-Dougan, Free, & RadkeYarrow, 1995). The association of maternal depression with maternal caregiving behavior and infant attachment is moderated by maternal representation of attachment relationships. Pearson, Cohn, Cowan, and Cowan (1994) found that women who had depressive symptoms exceeding clinical cutoff but had evolved a secure representation of attachment relationships engaged in caregiving behavior comparable to those mothers with a secure representation of attachment who were not depressed. Mothers' working models of relationships, based in part on their own understanding of childhood experiences with parents, affect their perceptions of infant emotions and their interpretations of their infants' behavior (Crockenberg & Leerkes, 2000) and influence their ability to be sensitive to their infants' signals, over and above their own mental health issues.

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As this review indicates, although emotional availability tends to be decreased and negative emotions of sadness and irritability tend to be increased when mothers who abuse alcohol and drugs or who are depressed interact with their infants, and their infants are more vulnerable to disruptions in attachment security, no single predominant pattern exists. Atypical mothers show varying degrees of dysfunction, and they and their offspring show a variety of developmental patterns.

VI. Conclusions The importance of early experience is shown in atypical as well as in normally developing children. Attachment relationships form, even in the most deviant groups: They form in autistic children, about whom a failure to attach was considered a criteria of the disorder in psychiatric diagnostic manuals (American Psychiatric Association, 1980, 1987), and they form in children with cerebral palsy who lack the ability to gain proximity to the caregiver, one of the defining features of attachment. Because attachment serves the biological purpose of ensuring human survival, there are probably many redundant processes ensuring its development and maintenance among typical and atypical infants and mothers. As a basic biological process that begins in the 1st year of life, minimal neurological and cognitive capacity on the part of the infant, in the context of everyday experience with a caregiver, probably are sufficient to promote attachment (Barnett et al., 1999). Mothers and infants show impressive flexibility as to behaviors used to fulfill a function, so that many neurologically impaired infants are able to participate in organized attachment relationships that fulfill the security need (Crittenden, 1999). Within groups of atypical children and atypical mothers, all forms of attachment relationships are possible and do occur, that is, those that are similar to relationships formed in typical dyads and those that differ. As with typical dyads, maternal sensitivity functions in atypical groups to increase the likelihood of attachment security. The association, albeit modest in strength, is evident among such disparate groups as infants born preterm, children with autism, children with Down syndrome, deaf infants, and infants of mothers who are depressed or who abuse alcohol and drugs. Despite muted and confusing affective signaling in Down syndrome, autistic children, and infants born very preterm, despite communicative difficulties with deaf infants, and despite motor handicaps with children with cerebral palsy, many mothers accept, accommodate, and are able to be sensitive to their infant, and many are able to provide experiences consistent with attachment security. As Goldberg (1988) concluded, "the propensity to form a secure attachment is not easily disrupted, and the majority of parents of ill and handicapped youngsters are sufficiently resilient to adapt to their situation in a fashion that provides the conditions for normal development of infant-mother relationships" (p. 185).

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Even among mothers whose cognitive and affective functioning is disrupted by depression or addiction to drugs and alcohol, many mothers are able to provide sufficient sensitivity to their infant's needs to furnish the basic experiences that lead to attachment security. The studies demonstrate that the construct and measurement of maternal sensitivity and the Strange Situation measurement of organization of attachment can be stretched to accommodate all groups. But methodological adaptations need to be made. Children with cerebral palsy who cannot locomote cannot gain proximity by their own efforts (Marvin & Pianta, 1996). Although proximity seeking is an important indicator of security in normally developing children, it cannot be used with children with cerebral palsy. Other indicators must suffice. Similarly, for autistic and Down syndrome children, neurological symptoms related to the disorders that mimic attachment disorganization behaviors must be differentiated from behaviors that arise in the relationship and do indicate disorganization in the relationship. Although secure attachment relationships can be and are formed by atypical children and by children of parents with psychopathology, the occurrence of attachment insecurity and attachment disorganization is increased, but not uniformly among these groups. Chronic medical conditions and neurological and physical handicaps may strain the quality of mother-infant interactions through increased neediness on the part of the child and increased stress and anxiety on the part of the parent, but they do not necessarily impede the mothers' sensitivity, that is, her ability to perceive and respond to infant cues. However, the deficits in verbal and nonverbal communication of children with autism and the ambiguity of their emotional signaling, the muted affect and ambiguous interactive cues of children with Down syndrome, and the diminished frequency of positive bids among infants born prematurely may well represent the range of children's characteristics that specifically impede mothers' abilities to perceive cues and interpret them accurately and thus limit their ability to respond consistently and sensitively. Alterations in children's responsiveness may have as much of an impact on maternal sensitivity as alterations in children's signaling. For example the decreased affective and social responsivity of children born very preterm, or those with autism or Down syndrome, and the increased negative affect in infants exposed prenatally to alcohol may limit shared positive affect within the dyad, may adversely affect mothers' sense of reciprocity in the relationship, and thereby may interfere with maternal sensitivity. Atypical groups, specifically children with autism and Down syndrome, also highlight the influence of children's cognition in fostering security in the attachment relationship. Attachment theory does not postulate that individual differences in infants' cognitive functioning influence maternal sensitivity or the quality of attachment in normally developing infants. Attachment theory proposes that a minimum level of infant cognition is needed to form a coherent working model

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of attachment. This level is a normal developmental achievement that may not be attained in severely cognitively impaired children. Or, alternatively, infant cognition may be a marker of the autistic and Down syndrome child's ability to signal clearly and to respond positively to their social environment and thereby affect maternal sensitivity. Research with atypically developing children thus teaches us to widen the examination of children's antecedents of maternal sensitivity to emphasize clarity and forcefulness of children's signaling, children's responsiveness, and their expression of affect, both positive and negative. Whereas contingent and appropriate responses to infant signals were the defining characteristic of sensitivity as intially proposed by Ainsworth and her associates (1978), the importance of the inclusion of mutuality and positive infant and maternal affect becomes highlighted in atypical groups. Although adults' response to negative affect in the child is crucial to the formation of attachment, diminished positive affect in the child may strain the formation of attachment security because it may diminish parental pleasure that may be important to fuel and maintain parental sensitivity. Maternal psychopathology also impedes the formation of security, but some forms of psychological disorder are more threatening to parental functioning, to maternal sensitivity, and to the attachment relationship than others. Substance abuse with paranoia or other antisocial personality disorders, or major depression with personality disorders of narcissism, schizotypal, and schizoid features, markedly interfere with emotional and attentional responsiveness. In contrast, maternal depressive symptoms occurring or during pregnancy in the first few months after birth that remit within the first 6 months postdelivery do not seem to have adverse consequences for the infant. Severe depressive symptoms that endure into infancy do. In understanding the effects of maternal mental health disorders on children's development, it is very important to consider the variability in the timing of the illness, the severity of the illness, and the presence of comorbid diagnoses. Research with atypical mothers underscores the powerful role that maternal affect plays in determining children's security or insecurity of attachment. Chronic severe negative affect impedes security. Disorganized attachments in typically developing children are associated with maternal frightening and frightened behavior in the presence of the child, which, in turn, are linked to mothers' lack of resolution of mourning and lack of resolution about experiences of physical or sexual abuse. Research has not yet been done to determine whether the increased rates of disorganization in atypical groups of children share this antecedent. But it is a likely cause in children of alcohol and drug-using mothers whose early histories typically are marked by experiences of abuse and/or significant losses of attachment figures through death. In summary, attachment relationships are established in most atypically developing children: those with hearing impairment or physical handicaps, or those who are chronically ill, or those born preterm, or those with cerebral palsy, autism or

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Down syndrome, or infants of mothers addicted to drugs or alcohol, or infants of mothers with depressive disorders. Attachments in these children can have the quality of security as in the majority of typically developing c h i l d r e n - - o r attachments can take different forms, forcing us to broaden our conceptual models of attachment formation. Atypical mothers and atypically developing children show us the power of affect expression and emotion regulation in determining the organization of attachment. Disorders that involve alterations in affect--ambiguity of emotional signaling, increased negative affect, diminished and muted positive affect, chronic f e a r f u l n e s s u i n either the mother or the child markedly increase the occurrence of insecurity and/or disorganization in the attachment. Although maternal sensitivity increases the likelihood of security among all children, only in atypically developing children do children's cognitive and affective skills so forcefully shape maternal sensitivity that the organization of attachment is directly affected.

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I N F L U E N C E S OF F R I E N D S A N D F R I E N D S H I P S " M Y T H S , TRUTHS, AND RESEARCH RECOMMENDATIONS

Thomas J. Berndt and Lonna M. Murphy DEPARTMENT OF PSYCHOLOGICAL SCIENCES PURDUE UNIVERSITY, WEST LAFAYETTE INDIANA 47907

I. INTRODUCTION II. INFLUENCES OF FRIENDS' CHARACTERISTICS A. MYTH: FRIENDS' INFLUENCE IS PREDOMINANTLY NEGATIVE B. MYTH: SOCIAL PRESSURE IS THE PRIMARY PROCESS OF INFLUENCE BETWEEN FRIENDS C. MYTH: FRIENDS POWERFULLY INFLUENCE CHILDREN'S ATTITUDES AND BEHAVIOR D. MYTH: FRIENDS' INFLUENCE REACHES A PEAK IN MIDDLE ADOLESCENCE III. INFLUENCES OF FRIENDSHIP QUALITY A. MYTH: MEASURES OF POSITIVE FEATURES ARE ADEQUATE FOR JUDGING FRIENDSHIP QUALITY B. MYTH: HAVING HIGH-QUALITY FRIENDSHIPS ENHANCES CHILDREN'S SELF-ESTEEM IV. INFLUENCES OF FRIENDS' CHARACTERISTICS IN FRIENDSHIPS DIFFERING IN QUALITY V. CONCLUSIONS AND IMPLICATIONS REFERENCES

I. Introduction Hypothesis 1- Friends have a significant influence on the attitudes and behavior of children and adolescents. Hypothesis 2: Supportive, high-quality friendships have a positive influence on the self-esteem of children and adolescents.

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Much of the research on friendships in childhood and adolescence has focused on one or both of these general hypotheses about the influences of friends and friendships. The hypotheses may seem like common sense rather than like propositions that need to be proven, but research suggests that the first hypothesis is true and the second hypothesis is a myth (Bemdt, 1999b; Keefe & Berndt, 1996). In both popular and scholarly writings on the influences of friends and friendships, myths and truths coexist to a surprising degree. One possible reason that the myths are so widely accepted is that adults think they can accurately remember their childhood experiences, when those memories have actually been altered by later experiences. Another likely reason is that questions about childhood friends and friendships become entangled with people's ideas about adolescence, a period of life about which misconceptions abound. Yet another reason is that some of the previous research on friendship and friends' influence has serious methodological flaws or has yielded findings that have been misinterpreted. Because many assumptions about the influences of friends and friendships have not been confirmed by empirical research, one aim of this chapter is to distinguish those that may be considered as truths from those that are myths. A second aim is to raise questions to which the correct answers are still unknown or uncertain and to suggest strategies for finding the answers. To achieve these two aims, the chapter includes some conclusions that can be drawn from previous research and some recommendations for future research. The chapter is divided into three sections of unequal length. In the first and longest, we focus on the influences of friends' characteristics. In other words, we examine how children's and adolescents' attitudes, behaviors, and other characteristics are influenced by the corresponding characteristics of their friends. Friends' characteristics represent one important pathway of friends' influence (Bemdt, 1992), and this pathway has been explored for decades by researchers from many disciplines. Many issues have been thoroughly investigated, and many conclusions can be drawn. In the second, shorter, section of the chapter, we focus on the influences of friendships. More specifically, we examine how children and adolescents are affected by having friendships that differ in positive features such as intimacy or in negative features such as conflicts. By assessing these features, researchers have tried to judge the quality of specific friendships. Friendship quality represents a second pathway of influence, one dealing not with the friends as individuals but with the relationship between friends (Bemdt, 1992). Research on friendship quality is little more than a decade old, so only a few conclusions about its influence can be drawn with confidence. However, some seemingly plausible hypotheses about friendship quality already appear to be myths. In particular, research has failed to confirm the hypothesis that high-quality friendships enhance adolescents' self-esteem. In the third section of the chapter, we focus on possible interactions between the influences of friends' characteristics and friendship quality. This section is brief

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because most researchers who have probed the influence of friends' characteristics have not assessed the relationships between those friends. Conversely, most researchers who have probed the influence of friendship quality have not assessed the characteristics of the individuals who are friends. However, questions about possible interactions between the two pathways of influence have great theoretical and practical significance. Most theories of interpersonal influence include some form of the hypothesis that children and adolescents are more influenced by the characteristics of friends when those friendships are higher in quality. If such interactions between friends' characteristics and friendship quality are not found, those theories will need to be revised. The practical significance of interactions between the two influence pathways can be illustrated with an apparently simple question: Is trying to increase the quality of children's or adolescents' friendships always a good idea? The answer would be yes, if high-quality friendships always have positive effects. The answer would be no, if the harmful influence of friends with negative characteristics is magnified when those friendships are higher in quality. We examine the evidence on this question, and on other types of interactions between friends' characteristics and friendship quality, in the third section of the chapter. Throughout the chapter, we emphasize studies of friendships among schoolage children and adolescents, because few researchers have investigated preschool children's friendships (but see Howes, 1996). Similarly, few researchers have investigated the changes in friendship between adolescence and adulthood (but see Furman & Buhrmester, 1992). Instead, researchers have generally focused on students in elementary schools, in middle or junior high schools, or in senior high schools. In this chapter, the age of the participants is mentioned in the description of specific studies. For simplicity, however, we use the term children to refer to both school-age children and adolescents when statements or issues apply to both.

II. Influences of Friends' Characteristics The hypothesis that children are influenced by their friends' attitudes and behaviors is hardly controversial. This hypothesis can be found in the philosophical and religious writings of authors from thousands of years ago. Nevertheless, four important issues concerning the influences of friends' characteristics are not well understood. The first issue is whether the predominant direction of friends' influence is positive or negative. In other words, does friends' influence generally lead to desirable or undesirable changes in children's attitudes and behaviors? The second issue concerns the processes by which friends influence each other. More specifically, is friends' influence primarily a result of the social pressure that friends exert on children, as Bronfenbrenner (1967, 1970) argued, or are other influence processes

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more important? The third issue concerns the magnitude of friends' influence. Do friends have a powerful influence on children, strongly determining the changes over time in their behavior? The fourth issue is how the magnitude of friends' influence changes with age. In particular, do the available data support the widely accepted idea (e.g., Steinberg, 1999) that friends' influence increases between childhood and adolescence but declines in late adolescence? A. MYTH:FRIENDS' INFLUENCEIS PREDOMINANTLYNEGATIVE A prominent theme in writings about adolescents is that they are negatively influenced by their peers. The peers who are such a negative influence on adolescents are not always specified, but many writers suggest that they are the members of the adolescents' friendship groups. For example, friends supposedly encourage adolescents to smoke cigarettes, drink alcohol, use drugs, and put little effort into their schoolwork. Certainly, some adolescents have friends who negatively influence their behavior. Some adolescents have friends who smoke cigarettes and pass cigarettes to their nonsmoking friends. Some adolescents have friends who bring alcoholic beverages to every social occasion and encourage friends to join them in drinking. And some have friends who invite them to spend time in social activities rather than in doing their homework or studying for school exams. The question, though, is whether most adolescents have friends who exert such a negative influence on their behavior. It is not surprising, perhaps, to think that they do. Several undesirable behaviors such as cigarette smoking, alcohol use, and the use of other illegal drugs increase during adolescence. Interactions with friends also increase during adolescence, and groups of friends often smoke cigarettes, drink alcohol, and use illegal drugs together. Nevertheless, these bits of evidence are not an adequate basis for the conclusion that friends' influence is the cause of the increase with age in negative behaviors. Likewise, these bits of evidence are not an adequate basis for the conclusion that friends' influence is predominantly negative. Before drawing these conclusions, researchers would need to supplement these general observations with data obtained using more sophisticated methods for assessing social influence.

1. Assessing the Direction of Friends' Influence Researchers have used several methods to assess friends' influence, but an especially powerful method is to see how changes over time in children's characteristics are related to the initial characteristics of their friends. Researchers conclude that friends' influence has been demonstrated if the children's characteristics become more similar over time to the initial characteristics of their friends. Using this method, Berndt and Keefe (1995) examined how students' attitudes, behavior, and achievement in school were influenced by their friends' attitudes, behavior, and achievement. The sample included approximately 300 seventh and

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eighth graders who initially were assessed late in the fall semester. The students reported on their positive involvement in classroom activities and on their disruptive behavior at school. In addition, the students named their three best friends. Two teachers of each student reported on the student's involvement, disruptive behavior, and report-card grades. Because most students named friends who were also participating in the study, the friends' scores for involvement, disruption, and behavior could be matched with the students' scores. The students' attitudes, behavior, and achievement were assessed again in the spring semester of the school year, 5-6 months after the first assessment. With this longitudinal design, the changes during the year in the students' adjustment to school could be evaluated. In addition, the relation between changes in students' adjustment and friends' adjustment in the fall semester could be determined. More specifically, Berndt and Keefe did hierarchical regression analyses in which each measure of the students' adjustment in the spring was the criterion variable in a separate analysis. The first predictor in every analysis was the same measure of the students' adjustment in the fall. The second predictor was the corresponding measure of the friends' adjustment in the fall. Using the fall measure of the students' adjustment as the first predictor controlled for the substantial continuity in students' adjustment during a school year. For example, students who are highly involved in classroom activities in the fall of a school year typically are highly involved in the spring semester. With this continuity taken into account, the results of the analyses can show what influenced the changes in students' adjustment during the year. In particular, if a measure of the friends' adjustment in the fall significantly predicts students' adjustment in the spring, then it is reasonable to conclude that the friends influenced the changes during the year in students' adjustment. Berndt and Keefe's (1995) analyses suggested that friends significantly influenced changes in some aspects of students' adjustment. For example, changes during the year in students' self-reported disruptive behavior were associated with their friends' level of disruptive behavior in the fall. One way to summarize these findings would be to say simply that friends influenced adolescents' disruptive behavior at school. However, such a summary could easily be misinterpreted. Because the label, disruptive behavior, refers to negative or undesirable behaviors, readers might erroneously conclude that these findings show that friends had an entirely negative influence on adolescents' behavior. Berndt and Keefe were careful to point out that their significant findings reflected two types of changes over time in students' disruptive behavior. First, students who initially had friends who were high in disruptive behavior increased their disruptive behavior. Second, students who initially had friends who were low in disruptive behavior decreased their disruptive behavior. The results of hierarchical regression analyses do not differentiate between these two types of changes. However, many researchers use their analytic technique to examine friends' influence on behaviors that are labeled negatively, such as drinking alcohol, using drugs, and engaging in delinquent behavior. And when the analyses suggest significant influences of

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friends, the researchers often describe their findings as showing that friends influence those behaviors. Readers of the research reports are very likely to infer that these reports bolster the argument for negative influences of friends by showing that friends promote increases (and never decreases) in drinking, drug abuse, and delinquency. On rare occasions, researchers have used data-analytic techniques that make it possible to distinguish between the negative and the positive influences of friends. For example, Epstein (1983) used grades and achievement-test scores of students in the fourth through eleventh grades as indicators of their academic achievement. She also obtained the names of the students' friends, and linked the achievement scores of students and their friends. Next, she divided the students in each grade into those who were either relatively high or relatively low in achievement. Then she determined whether these students had friends who, on the average, were relatively high or relatively low in achievement. One year later, she assessed the students' achievement again. Epstein found that the groups of students whose friends' average level of achievement was different from their own became more similar to their friends over time. That is, the students who initially were relatively high in achievement but who had friends relatively low in achievement decreased in their achievement. Conversely, the students who intially were relatively low in achievement but who had friends relatively high in achievement increased in their achievement. Most important, the positive influence of high-achieving friends seemed to be as great as the negative influence of low-achieving friends. The predominant direction of friends' influence might still be predominantly negative if most children chose friends whose social, psychological, and academic adjustment was worse than their own. For example, friends would have a largely negative influence on children's academic achievement if most children chose friends lower in achievement than they were themselves. Yet this hypothesis can be rejected, because decades of research have shown that children choose friends who are similar to themsevelves in achievement and in many other attributes (McPherson, Smith-Lovin, & Cook, 2001). Moreover, when children seek friends different from themselves, they typically aim for friends higher in achievement and in other positive attributes than they are themselves (e.g., Savin-Williams, 1987). The tendency of children and adolescents to choose friends whose adjustment is as good as or better than their own explains other findings that are inconsistent with popular beliefs about friends' influence. Most adolescents report that their friends put more pressure on them to remain positively involved in school than to be uninvolved (Clasen & Brown, 1985). Most adolescents say that their friends disapprove rather than approve of cigarette smoking (Urberg, Shyu, & Liang, 1990), and most say that their friends discourage rather than encourage drinking alcohol (Keefe, 1994). This evidence is so consistent and so contrary to popular belief that it deserves to be stated as the first truth about friends' influence:

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Truth: Duringchildhoodand adolescence,friendsgenerallyencouragesociallydesirable behaviors.

2. Taking a Theoretical Perspective: Similarity and Group Shifts The evidence reviewed thus far either fails to confirm or directly disconfirms the hypothesis that friends' influence is predominantly negative in childhood and adolescence. But is it appropriate to call this assumption about friends' influence a hypothesis? Calling it a hypothesis raises a question about the theory from which it derives. In fact, there is none. Stated more fully, the literature on friends' influence is almost entirely atheoretical, and very few researchers have explicitly discussed whether it makes sense to assume that friends' influence could be biased in a negative direction. Researchers who have linked questions about friends' influence to general theories of social influence have invariably reached a different conclusion (e.g., Hartup, 1996). These theories include the implicit or explicit hypothesis that social influence makes people's characteristics more similar to those of their relationship partners. Thus, a partner's influence may increase positive or negative behaviors, depending on the partner's behaviors. Consequently, the idea that friends have a predominantly negative influence should be considered as no more than a myth. That idea should be replaced by the following: Truth: Rather than being largely negative, the direction of friends' influence is usually to make children's characteristics more similarto their friends' characteristics. In the preceding statement, the word usually does not reflect only the caution of researchers and scholars about making absolute statements. It also reflects a tradition of research on social influence in which similarity is not the typical outcome of influence processes. In the 1960s, social psychologists began research on what was first known as the risky shift but later was given the more general label of group polarization (Isenberg, 1986). This research was inspired by a few studies in which small groups of adults were asked to make a decision that involved risk. For example, they were told about a person who had to choose between two jobs, one with high security but a low salary and another with a high salary but low job security. After a group discussion, adults often chose more risky alternatives than the average position of the group members before the discussion. In other studies with other types of decisions, groups sometimes chose more conservative alternatives than the average positions of the group members before their discussion. Over time, researchers discovered that the effect was not limited to discussions of decisions regarding risk versus caution but was applicable to a wide range of decisions. When the average positions of group members were biased in one direction before discussion, that bias usually increased after discussion, so the final group decision was more extreme than the initial opinions of the individuals in the group would have suggested. In short, discussions seemed to push group decisions toward one

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extreme or the other, making different groups more polarized than they were initially. Very few researchers have examined the phenomenon of group polarization with children or adolescents. In one study (Berndt, McCartney, Caparulo, & Moore, 1983-1984), groups of four students in the third grade or in the sixth grade discussed dilemmas involving honest behavior and altruistic behavior. The discussions led to shifts in students' decisions: Groups shifted toward more altruistic choices on the altruism dilemmas and toward more dishonest choices on the honesty dilemmas. However, contrary to research on group polarization in adults, the initial bias in a group's opinions had relatively little impact on the direction or degree of change in decisions. Another study involved discussions between pairs of eighth graders who were close friends (Berndt, Laychak, & Park, 1990). The friends discussed hypothetical dilemmas related to achievement motivation. For example, they had to decide whether to spend an evening at a rock concert or to stay home and study for an important exam at school the next day. Before and after the discussions, the students made independent decisions about the dilemmas. The discussions resulted in shifts in the average decisions of the pairs of friends, but the shifts were sometimes toward more neutral rather than more polarized decisions. The direction of the shifts was related to the arguments made and the information exchanged during the discussions. Also analyzed was whether the decisions by the two friends became more similar after their discussions, and these analyses confirmed that the discussions did increase the similarity of friends' decisions. Obviously, two studies of children's discussion do not provide a basis for any conclusions. However, the studies are provocative, in part because they illustrate an experimental paradigm for examining friends' influence. Thus they lead to a recommendation for future research: Research recommendation: Examine the effects of friends' discussions, seeking to explain both increases in friends' similarity and group shifts.

Additional studies of friends' discussions would be valuable not simply because they allow the exploration of friends' influence with a fully experimental design. They are also valuable because they can illuminate a phenomenon that may be of great practical significance. Sometimes groups of individuals decide on joint actions that none of the individuals would have done alone. The scientific literature on such cases is diverse, ranging from studies of groupthink to studies of mob psychology and gang behavior (e.g., Janis, 1982; Thrasher, 1927). For example, Thrasher's (1927) classic monograph on Chicago gangs includes many examples of adolescents engaging as a group in acts of burglary and theft that they would have been unlikely to attempt as individuals. This literature suggests that the causes of single episodes of collective action may differ from the causes of

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long-term changes in individuals' attitudes and behavior. The longitudinal studies described earlier in this section are valuable in showing to what extent long-term changes in children's attitudes and behavior are a consequence of friends' influence. Experimental studies of friends' discussions are valuable in understanding the immediate influence of a group of friends on an individual's decisions. B. MYTH: SOCIAL PRESSURE IS THE PRIMARY PROCESS OF INFLUENCE BETWEEN FRIENDS

In groundbreaking research, Bronfenbrenner (1967, 1970) argued that peers have influence because they put pressure on children to change their behavior. Bronfenbrenner was referring to the influence of all of a child's peers, which might include all the child's classmates or perhaps an even larger group. Nevertheless, the idea that peer pressure is the primary process of influence between friends has been widely accepted. Moreover, the idea that friends' influence depends on peer pressure has often been linked, as it was by Bronfenbrenner (1970), to the idea that the direction of friends' influence is predominantly negative. Based on these arguments, intervention programs were designed to train children to resist the negative pressure they supposedly were receiving from peers (see Cook, Anson, & Walchli, 1993). Many types of research have shown, however, that the idea of a group of friends putting pressure on a single child to do whatever they want the child to do is a myth. First, ethnographic research has shown that peers rarely put pressure on children in a group to conform to some standard for behavior (Sherif & Sherif, 1964; Suttles, 1972). In adolescents' groups, decisions about what to do together are generally made after an informal discussion in which each person has an opportunity to influence the others and to be influenced by the others. One member of the group is likely to be more influential than others, but not even that person can force the others to do what he or she wants. Force is unlikely because friendship groups are voluntary. Faced with pressure to conform, an adolescent can simply choose to leave the group. Second, children themselves report that they do not face much pressure from peers to do things they do not want to do (Ansley & McCleary, 1992). When talking about their relationships with friends, children are even more definite in saying that friends must accept and respect each other (Berndt, Miller, & Park, 1989; Rawlins, 1992). In particular, friends must recognize one another's rights to think differently or to choose different activities than they do. Long ago, Piaget (1932/1965) made similar comments about all relationships among peers. He said that peer relationships in middle childhood and adolescence are based on mutual respect. Peers understand that they cannot reach agreement if one individual insists on the fight to make a decision for all; agreement can be reached only if everyone listens to everyone else's opinions and then seeks a

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solution satisfactory to all. This portrait of peer relationships may seem idealistic, but children know that if they are not happy with how their group of friends treats them, they are always free to say, "I won't be your friend anymore." Third, when pressure is applied in peer groups or between friends, it is usually a sign of conflict rather than a prelude to agreement. In the experimental study of friends' discussions mentioned earlier (Berndt et al., 1990), less change in the friends' independent decisions occurred when discussions were high in conflict than when they were harmonious. Similar results have been found with peers' discussions of moral dilemmas (Damon & Killen, 1982). Fourth, a few of the programs designed to increase adolescents' skills in resisting peer pressure have been evaluated rigorously (Cook et al., 1993). The programs were intended to reduce drug use in adolescence, and the evaluations indicated that the programs were successful. However, their success was less closely related to their effectiveness in teaching the adolescents to resist peer pressure than to their ability to convince adolescents that their peers generally did not use drugs and did not want to do so. That is, the programs were effective because they changed adolescents' perception of their peers' norms regarding drug use, not because they made adolescents less susceptible to peer pressure. These findings are not at all surprising if, indeed, social pressure is not the usual way that friends and other peers influence adolescents' behavior. If not social pressure, then which processes are primarily responsible for friends' influence? During childhood and adolescence, friends' influence depends on the same processes that explain social influence at other ages. One important process is social reinforcement, and the effects of peer reinforcement have been demonstrated in many laboratory experiments (Hartup, 1983). The power of peer reinforcement in natural settings was vividly demonstrated by Dishion and his colleagues (e.g., Dishion, McCord, & Poulin, 1999; Dishion, Poulin, & Burraston, 2001; Dishion, Spraclden, Andrews, & Patterson, 1996). They found that boys high in antisocial behavior often laugh or provide similar positive responses when a friend jokes about behaviors that are deviant or that break social rules. Moreover, the greater the amount of reinforcement, the greater the escalation over time in the boys' substance use, aggressive behavior, delinquency, and high-risk sexual behavior. In short, Dishion's research confirms that friends need not pressure children to engage in deviant behavior. All friends need to do is make that behavior seem exciting and enjoyable. Social learning theory suggests the importance of observational learning among friends (Bandura, 1977). Learning from peer models has been demonstrated in many laboratory experiments (Hartup, 1983; Schunk & Zimmerman, 1997), and occurs in natural settings as well (Berndt, 1999a). A negative exampls is when an adolescent sees a friend smoking a cigarette and decides to try one too. A positive example is when an adolescent hears from a friend that he or she will spend the evening studying for a school exam and decides to do the same.

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Another process that is rarely mentioned in connection with friends' influence is rational decision making through information exchange. With only one exception, theorists who have considered the phenomenon of peer influence have emphasized social and emotional processes and ignored cognitive processes. The exception is Piaget (1932/1965), perhaps the only prominent psychologist of the 20th century who had an unequivocally positive view of peer relationships. (Ironically, he was perhaps the only prominent psychologist in that century who had an unequivocally negative view of parent-child relationships!) Piaget argued that the mutual respect in peer relationships arises from peers' ability to understand one another's perspectives. This understanding improves during discussions with peers, and increased perspective-taking leads to greater maturity in moral reasoning. Similar hypotheses about the benefits of discussions among friends have been tested and largely supported in research inspired by Kohlberg's extension of Piaget's theory of moral development (e.g., Walker, Hennig, & Krettenauer, 2000). Another cognitive explanation of group influence has been thoroughly examined by researchers who have tried to understand the phenomenon of group polarization (e.g., Vinokur & Burnstein, 1978). These researchers emphasize the effects of persuasive arguments on the shifts that occur in group decisions after discussion. Their research has shown that group shifts can usually be predicted from a content analysis of the number of arguments for each decision that are expressed during a discussion and observers' ratings of the persuasiveness of those arguments. Berndt and his colleagues tested a simplified version of this model in his experimental studies of decision making in four-person groups (Berndt et aL, 1983-1984) and in pairs of friends (Berndt et al., 1990). In each study, analyses of the information exchanged during the discussions showed that group shifts occurred in the direction supported by the greatest number of arguments. In short order, then, this survey of influence processes has covered the spectrum from coercive pressure at one extreme to something very close to a reasoned debate at the other extreme. Many strands of evidence indicate that coercive social pressure is a rarely used and rarely effective technique of influencing friends. More commonly used and more effective are processes of reinforcement, observational learning, and information exchange. This conclusion can be summarized as follows: Truth: Many processes are involved in friends' influence, but social pressure is uncommon. Weaknesses in the research base supporting this conclusion about influence processes must be acknowledged, however. The preceding survey of specific processes and their effects was not a selection of illustrative examples from a much larger literature. Unfortunately, we have cited much of the data available, particularly about children and their friends. Too often, those data come from laboratory experiments exclusively, from a single research program, or from fewer than a handful

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of studies. These gaps in the literature lead to the following recommendation for research: Research recommendation: Use observational and experimental studies to increase understanding of processes underlying friends' influence.

The value of observational studies is demonstrated by the research on the reinforcement of deviant behavior during antisocial boys' interactions with friends (Dishion et al., 2001). The value of experimental studies is demonstrated by the research on group discussions (Berndt et al., 1983-1984, 1990). More research with similar methods would add substantially to our understanding of exactly how friends influence one another during childhood and adolescence. That research would, in turn, provide a surer foundation for interventions designed to enhance the positive influence and reduce the negative influence of friends on children's behavior. C. MYTH: FRIENDS POWERFULLY INFLUENCE CHILDREN'S ATrlTUDES AND BEHAVIOR

Assertions about how friends pressure children to behave are often coupled with assertions about how powerful the influence of friends is. Many writers have stated, often with great alarm, that friends have a very strong influence on children's and adolescents' behavior (e.g., Bishop, 1989). Unfortunately, many studies of friends' influence contain serious methodological flaws that lead to inflated estimates of friends' influence. For several decades, thoughtful researchers have repeatedly pointed out these flaws (e.g., Kandel, 1978, 1996). However, the same flaws continue to be found in published studies, even in the most selective journals, so they must be pointed out again.

1. Friendship Selection Versus Friends'Influence One major flaw is the use of a correlation between children's characteristics and their friends' characteristics as an indicator of the friends' influence on the children's characteristics. At first glance, this practice may seem perfectly reasonable. A correlation provides one estimate of the similarity between two variables, and one of the truths stated earlier was that the usual result of friends' influence is an increase in friends' similarity. Where's the problem? The problem is that friends' similarity is not always the result of their influence on one another. Two individuals may become friends because they are already similar to each other. Moreover, the formation of friendships between children who are already similar may result not only from a conscious selection process but also from other circumstances that create opportunities for friendship formation (McPherson et al., 2001). For example, friends tend to be approximately the same age, undoubtedly because children's classmates tend to be the same age and being

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in the same class gives students many chances to get to know one another and then to become friends. Researchers use the term selection to refer to all processes, conscious or not, that cause children who are similar in certain characteristics to become friends. Selection can explain a substantial part of the similarity that exists between friends on characteristics that have been the focus of research (Fisher & Bauman, 1988). Friends may be similar in cigarette smoking, for example, because all smokers gather in the same place on the school grounds to smoke, and all nonsmokers avoid that location. Friends may be similar in alcohol use because those who want to drink alcohol attend the parties at which they know alcohol will be available, while those who do not want to drink stay away. Kandel (1978, 1996) has argued that approximately half of the variance represented by a correlation for best friends' similarity is due to selection rather than influence. By contrast, Cohen (1977) estimated that substantially more of the similarity among adolescents in a friendship group is due to selection than to influence. These contrasting estimates are difficult to evaluate because the two researchers examined friends' similarity on different characteristics and used different methods to assess selection and influence. Unfortunately, few other researchers have tried to estimate the relative importance of selection and influence for friends' similarity. What is clear, though, is that friends' similarity at one point in time provides little information about the friends' influence on one another. Thus, the next recommendation is: Research recommendation: Do not use similarity at one time to estimate friends' influence. Of course, negative recommendations are normally less useful than positive ones. The positive version of the recommendation would be to conduct either longitudinal studies or experimental studies to assess friends' influence. Unfortunately, neither of these designs can provide precise estimates of friends' influence in natural settings. Longitudinal studies done over periods of a few months or a year can only show how much influence friends had on changes in children's characteristics over that period. Moreover, they are imperfect for that purpose, because some children change their friends between times of assessment, and the combined influences of their old and new friends are impossible to assess. Therefore, the findings of longitudinal studies probably underestimate the influences of friends. Experimental studies involve the manipulation of participants' experiences, so they cannot do much to clarify the strength of friends' influence in natural settings. Moreover, experimental studies of discussions between friends may partly obscure the full array of processes by which friends influence one another. Consequently, the value of these studies is less in showing how much friends actually influence each other than in increasing understanding of one specific way in which they do so.

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2. Children's Reports on Friends Versus Friends' Self-reports Another major flaw in the research on friends' influence is using children as informants both on their own attitudes and behaviors and on their friends' attitudes and behaviors. In some studies, this flaw is coupled with the previous flaw of using a correlation for friends' similarity at one time as an index of friends' influence. That is, researchers estimate the strength of friends' influence from the size of the correlation between children's reports on their own characteristics and their reports on their friends' characteristics (see Ide, Parkerson, Haertel, & Walberg, 1981). But even when researchers use a longitudinal design to determine how much friends' influence on one another increases their similarity, having children report both on their own characteristics and on their friends' characteristics leads to seriously inflated estimates of friends' influence (Kandel, 1996). For example, Table I shows some estimates of friends' influence obtained in the study described earlier by Berndt and Keefe (1995). Besides reporting on their own involvement in school and disruptive behavior, students reported on their friends' involvement and disruption. The friends' involvement and disruption could also be judged from their reports on themselves, because of the matching of students with friends that was noted earlier. Table I shows the standardized regression coefficients from hierarchical regression analyses in which students' involvement or students' disruption in the spring of the year was the criterion, students' involvement or disruption in the previous fall was the first predictor, and friends' involvement or disruption was the second predictor. Only the results for the second predictor are shown in the table, because those are most relevant. For the analyses of students' reports on friends' involvement and disruption, the results are shown for the students' very best friend and for the average of three best friends. For the analyses of the friends' actual reports, results are shown for the TABLE I Standardized Regression Coefficients from Analyses in Which Students' Reports on Friends and Friends' Self-reports Were Predictors of Students' Adjustment in the Spring of a School Year, with Students' Adjustment in the Previous Fall Statistically Controlled Friends' measure Adjustment measure

Very best friend

Classroom involvement Students' reports on friends Friends' self-reports Disruptive behavior Students' reports on friends Friends' self-reports *p<.05.

**p<.01.

***p<.001.

.17"** .03 .12* .12*

Multiple friends

.17"** -.02 .24"* .15" *

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influence of the closest friend with whom each student could be matched and for the average of all the friends (up to three) with whom a student could be matched. Table I shows that the estimates of the influence of the very best friend are generally comparable to the estimates of the influence of multiple friends. More important, for very best friends and for multiple friends, the estimates of friends' influence are generally higher for measures derived from students' reports on friends than for measures derived from the friends' self-reports. For involvement and disruption, the estimates derived from students' reports on friends are highly significant and those derived from the friends' self-reports are close to zero. The difference exists because children believe they are more similar to their friends than they actually are (Aboud & Mendelson, 1996). When children are asked how their friends think and act, they largely report how they themselves think and act. Thus, they project their own attitudes and behavior onto their friends. Because of the similarity between friends, projection may sometimes lead to partly accurate reports on friends. However, for some characteristics such as sexual attitudes, children's reports on friends may be mostly or completely inaccurate (Wilcox & Udry, 1986). Despite their flaws, measures of friends' characteristics that derive from children's reports on friends abound in the literature. Children have been asked to report how many of their friends smoke, how often their friends drink alcohol, whether their friends use marijuana, and so on. Even more troubling is the common practice of mentioning the use of children as informants on friends only in a brief note in the Methods sections. The Abstract, Introduction, Results, and Discussion sections are written as if the researchers directly assessed the friends' smoking, drinking, drug use, or whatever behavior is under investigation. This practice is seriously misleading. Obtaining reports directly from friends is not an easy matter. A researcher must first get precise information on the identity of students' friends, then make certain that a substantial number of those friends are also research participants, and then manipulate the data to match students' scores with their friends' scores. These tasks can be logistically challenging and technically complex. By contrast, obtaining children's reports on friends is trivially easy. A single questionnaire can be given to a large number of children that includes items about both the children and their friends. This procedure is very economical, but it is a false economy. At a minimum, this method exaggerates the magnitude of friends' influence, but it could also distort research findings in other ways. For example, the accuracy of children's reports of their friends' characteristics may differ for boys and girls (e.g., Berndt & Keefe, 1995) or for children of different ages. Consequently, analyses of these reports may lead to incorrect conclusions about gender and age differences in friends' influence. Problems exist not only with measures based on children's reports of their friends' characteristics, but also with measures based on other types of reports

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about friends. Children may be asked, for example, whether their friends would approve if the children drank beer. They may be asked whether their friends have ever offered them a cigarette. Measures of this type can be useful, but the burden of proof should be on the researchers to demonstrate that the measures are valid. For example, are children's reports of their friends' approval of drinking beer strongly correlated with the friends' reports of their attitudes toward drinking? Are children's reports of cigarette offers from friends strongly correlated with the friends' reports of offering cigarettes to them? Without this evidence, a plausible argument can be made that the so-called friends' measures are actually proxy measures of the children's own attitudes and behaviors. Thus, the recommendation for future research is: Research recommendation:Only use children's reports on friends to judge the friends' characteristics when the validityof those reports can be demonstrated. Although the available data suggest that children's reports on friends have serious limitations, researchers may in the future find ways to overcome those limitations. To begin with, researchers may devise questions that call for less inference by children. If the topic is cigarette smoking, for example, children might be asked specific questions about each of their close friends (e.g., "Have you ever seen Susie smoke a cigarette? .... Do you know if she smokes cigarettes regularly, at least once a day?") Researchers might also argue that some reports on friends provide important data in themselves, because they indicate children's perceptions of the norms in their friendship groups. A convincing argument of this sort probably could be made, if it was linked to a conceptual framework that clarified the significance of perceptions of friends' norms. For example, we noted earlier that changing adolescents' perceptions of their peers' drug use can contribute to a decrease in their own use of drugs (Cook et al., 1993). Researchers evaluating other interventions might, therefore, want to ask children to report on their friends' characteristics even if those reports are known to be inaccurate. 3. How Strong Is Friends'Influence ? To summarize the preceding sections, the strength of friends' influence is exaggerated whenever it is estimated by a correlation for friends' similarity at one time or by using children's reports on friends as measures of friends' characteristics. But when these methodological flaws are absent, how influential are friends? The results in Table I for measures based on the friends' self-reports might seem to answer that question. The small regression coefficients suggest that friends' influence is weak, but that it is stronger for disruptive behavior than for classroom involvement. Similar results have been found in other short-term longitudinal studies. For some measures in some studies (e.g., Trernblay, Masse, Vitaro, & Dobkin, 1995), the coefficients did not even reach statistical significance. Taken together, these

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findings suggest that the idea that friends have a powerful influence on children's behavior should be dismissed as a myth. This conclusion can be stated as follows: Truth: Friends' influence is modest and varies for different attitudes and behaviors.

Would it be sensible to go even further, and reject the hypothesis that friends have a significant influence on children's attitudes and behaviors? Probably not, for several reasons. First, as mentioned earlier, estimates of friends' influence from longitudinal studies only suggest how much friends influenced the changes in children's characteristics over the short period between times of measurement. Moreover, because some children must have ended old friendships and made new friends during that period, those estimates must certainly be too low. Second, the atheoretical character of research on friends' influence was mentioned in connection with the issue of the direction of friends' influence, but it is equally relevant to the question of its strength. Children spend varying amounts of time with friends, but some spend hours with friends daily. Moreover, friendships normally last for months or years. Would any theory of social influence suggest that relationships of this kind would not have a significant influence on those involved? The answer is definitely no, so an argument that friends have little or no influence on children would simultaneously be an argument against all theories of social influence. Even so, serious debates about the magnitude of friends' influence are heuristically valuable because they can point research in new directions. We have argued that the myth of powerful friends' influence should be replaced by the truth that influence is modest and varies for different attitudes and behaviors. Such variations have received much less attention than they deserve. Understanding these variations would certainly contribute to the refinement of theories of friends' influence. Moreover, that understanding would be extremely important in planning interventions to minimize the negative influence of friends on children. Therefore, we end this section with another recommendation for research: Research recommendation: Assess and try to understand the variations in the strength of friends' influence on different attitudes and behaviors.

D. MYTH: FRIENDS' INFLUENCE REACHES A PEAK IN MIDDLE ADOLESCENCE

Bronfenbrenner (1967, 1970) inaugurated modem research on peer influence when he developed a set of hypothetical dilemmas in which children were asked how they would respond if peers encouraged them to engage in antisocial behaviors such as cheating on a test. Bronfenbrenner's research was highly visible and provocative because he showed that adolescents in the United States were especially likely to choose the antisocial behaviors if they thought that their peers

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would see their answers. By contrast, adolescents in the former Soviet Union were unlikely to choose those antisocial behaviors if they thought their peers would see their answers. A few years later, Berndt (1979) adapted Bronfenbrenner's antisocial dilemmas and created new dilemmas involving both neutral and altruistic behaviors. One version of the dilemmas referred specifically to encouragement from close friends rather than from peers in general. When children and adolescents responded to the adapted and new dilemmas, choices of the alternatives supposedly encouraged by friends increased steadily between third and ninth grade and then decreased slightly by twelfth grade. Thus, conformity to peers seemed to peak in midadolescence. This conclusion now seems highly doubtful. One reason for these doubts is that problems with the use of hypothetical dilemmas to measure conformity to peers are more apparent than they were in the late 1970s (Berndt, 1997, 1999a). First, conformity refers to shifts in children's attitudes or perceptions in the direction of some socially imposed norm (Hartup, 1970). When children respond to the hypothetical dilemmas, however, they need not change their attitudes or perceptions. They may, instead, simply report their willingness to join friends in the activities described. Second, during the long period since Bronfenbrenner's original publication on the antisocial dilemmas, no researcher has provided any evidence for the convergent validity of the dilemma measures. That is, no researcher has shown that responses to the dilemmas are correlated with measures of conformity to friends, or susceptibility to friends' influence, obtained using other methods. The uncertain validity of the dilemma measures is of special concern because consistent age trends in susceptibility to friends' influence have not been found in other types of studies. In particular, longitudinal studies of friends' influence that included participants varying in age have yielded inconsistent results. Age changes in friends' influence were absent in one study of cigarette smoking and alcohol use by sixth, eighth, and tenth graders (Urberg, Degirmencioglu, & Pilgrim, 1997). In another study (Urberg et al., 1990), friends' influence on cigarette smoking appeared to be greater at eighth grade than at eleventh grade. In a third study (Keenan, Loeber, Zhang, Stouthamer-Loeber, & Van Kammen, 1995), friends' delinquent behavior appeared to influence disruptive and delinquent behavior equally at fourth and seventh grades. Yet another study showed that friends' influence on adolescents' educational aspirations was greater at twelfth grade than at tenth grade (Hallinan & Williams, 1990). This pattern of results defies explanation. The argument might be made that just as the strength of friends' influence varies for different attitudes and behaviors, so might the age changes in its strength. Moreover, plausible explanations for these variations are sometimes obvious. For example, friends' influence on adolescents' educational aspiration may increase between tenth and twelfth grade (Hallinan & Williams, 1990) because decisions about college are more salient to twelfth graders

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than to tenth graders. Friends may therefore talk more about college decisions and so have more influence on each other's decisions in twelfth grade than in tenth grade. However, further research may show that age changes in friends' influence are not consistent even for different studies that focus on the same behavior. Already noted were two studies of friends' influence on adolescents' cigarette smoking done by the same research team that revealed different age trends (Urberg et al., 1990, 1997). Moreover, decades of research on the related topic of conformity to peers have not revealed a consistent developmental trend for peer conformity (Berndt, 1999a). If additional longitudinal studies of friends' influence yield equally inconsistent results, researchers may be forced to abandon the hypothesis that the strength of friends' influence on specific attitudes and behaviors changes systematically as children move into and through adolescence. Yet once again, referring to the idea of developmental changes in friends' influence as a hypothesis raises a question about the theory from which the hypothesis derives. Again, the answer is none. No theory of social influence includes specific hypotheses about developmental changes in susceptibility to friends' influence. Hypotheses about developmental changes in friends' influence might deserve consideration if future research reveals consistent age changes in friends' influence on specific attitudes or behaviors. Alternatively, hypotheses of this kind might be part of a new theory of friends' influence in childhood and adolescence. For now, though, the myth of a peak in friends' influence in middle adolescence must be replaced by the following negative conclusion: Truth: Consistent age changes in the strength of friends' influence have not been found.

When developmental researchers do not find consistent age changes in some phenomenon, they often switch to the exploration of individual differences. Such a switch could greatly increase understanding of friends' influence because individual differences have been virtually ignored in previous research. In particular, researchers have not investigated which attributes of children make them most susceptible to friends' influence and which attributes make children most able to influence their friends. The practical significance of this type of research is obvious, but its theoretical significance is equally great. A first step in this direction would be to connect theories and research on friends' influence with theories and research in two domains where individual differences have been a focus for generations: personality (Caspi, 1998) and intellectual abilities (Ferrari & Steinberg, 1998). For example, children who are relatively anxious and relatively unassertive may be especially susceptible to friends' influence. By contrast, children who are relatively outgoing may be especially able to influence their friends. In addition, children who are relatively intelligent might be low in susceptibility to influence attempts (see Wood & Stagner, 1994) while having a high ability to influence others.

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Other hypotheses about the relations of personality traits and intelligence to individual differences in influence could easily be generated. Then measures of personality and intelligence could be included in longitudinal and experimental studies of friends' influence. The planning of this type of research may be challenging, but the goals of the research are easily specified in the following recommendation: Research recommendation: Assess which children are most susceptible to friends' influence and which children have the most influence on their friends.

III. Influences of Friendship Quality Moving from questions about friends' influence to questions about the influences of friendships may seem simple, but it actually requires a radical change in perspective. Studies of friends' influence focus on individuals and their characteristics. Studies of the influences of friendships focus on the relationships between individuals. Even more striking is the difference in presuppositions about the outcomes of influence. The central question in research on friends' influence has been whether children's attitudes and behaviors are negatively affected by their interactions with friends. By contrast, the central question in research on friendships has been whether children's social and psychological adjustment are positively affected by having high-quality friendships. Consequently, when examining the influences of friendship the first issue to consider is how the quality of children's friendships should be defined and assessed. A. MYTH:MEASURES OF POSITIVEFEATURES ARE ADEQUATE FOR JUDGING FRIENDSHIPQUALITY The origins of research on friendship quality can be traced to the writings of Sullivan (1953) about preadolescent friendships. According to Sullivan, friendships first become close relationships in the years just before puberty. More specifically, in adolescence friends begin to share their most personal and private thoughts and feelings with one another. Sullivan labeled friendships characterized by this deep level of self-disclosure as intimate, and intimacy has since then been considered the hallmark of a high-quality friendship. Later studies of children's ideas about friendships revealed that children consider other features of friendships besides intimacy as important (Berndt, 1986). Children consider prosocial behavior (e.g., helping, sharing) as a positive feature of friendship. Children expect friends to be loyal, for example, by sticking up for them when they are in an argument with other peers. Children expect friends to be faithful, not to leave them for someone else or to have a party and not invite them.

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The positive features of good friendships overlap greatly with the features of relationships emphasized in theories of social support (Berndt, 1989; Uchino, Uno, & Holt-Lunstad, 1999). Those theories emphasize the contributions of close relationships to the enhancement of self-esteem. In particular, friendships foster self-esteem when the friends praise one another for their accomplishments and encourage one another when they are feeling bad about themselves. Several teams of researchers have devised interviews or questionnaires to assess the positive features of children's friendships (see Furman, 1996). For example, to assess a friendship's intimacy, children are asked, "When something is bothering you, how often do you talk to [friend's name] about it?" To assess prosocial behavior, children are asked, "How often does [friend's name] help you when you can't do something by yourself?" To assess self-esteem support, children are asked, "When you think you are not doing well in school, sports, or something else, how often does [friend's name] make you feel better about yourself?" Children who describe a friendship as higher in one positive feature such as intimacy usually describe the friendship as higher in other positive features as well (Berndt & Perry, 1986). Therefore, children's reports on all positive features can be combined into a single measure of positive friendship quality. Such a measure can also be defined as showing the degree to which a friendship is a supportive relationship. Friendship researchers initially ignored or gave little attention to negative interactions with friends. The implicit assumption seemed to be that children's best friendships must differ only in how positive they are, because children would end friendships that had negative features. That assumption must be placed in the category of myth. Two types of negative interactions commonly occur between friends; those types of interactions define two negative features of friendships. First, children understand that disagreement, arguments, and other kinds of conflicts can occur between friends (Berndt, 1986). Children mention the existence of conflicts between friends when asked about friendship in the abstract. Children also talk openly about the conflicts in their own friendships. For example, some children say that they and their friends often "get into arguments" or "annoy and bug each other." Second, children understand that friendships are not always marked by the equality and mutual respect that Piaget (1932/1965) believed was found in all peer relationships. On the contrary, friendships may involve frequent struggles over dominance. Children sometimes say that a friend tries to boss them around or to insist that they do what the friend wants them to do. Friendships may also be high in unpleasant forms of competition and rivalry. Children say that some friends try to show off their superiority to them or the friends brag about doing something better than they do. Dominance struggles, unpleasant competition, and rivalry can be described collectively as signs of inequality in a friendship.

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Most children who report more frequent conflicts with a friend also describe that friendship as higher in the signs of inequaltiy. Thus, reports of conflicts and inequality can be combined into a measure of negative friendship quality. Such a measure could also be defined as showing the degree to which a friendship is a source of interpersonal stress. Friendships higher in negative features tend to be lower in positive features, but this correlation is fairly weak (see Berndt & Keefe, 1995). The weak correlation indicates that positive and negative features of friendships should be examined separately. This conclusion can be stated as follows: Truth: Measures of both positive and negative features are needed to assess friendship quality.

B. MYTH: HAVING HIGH-QUALITY FRIENDSHIPS ENHANCES CHILDREN'S SELF-ESTEEM

Sullivan's (1953) hypothesis that high-quality friendships enchance children's self-esteem has been endorsed by many researchers, including those who view friendships as supportive relationships (Uchino et al., 1999). Evidence consistent with the hypothesis has been obtained in many correlational studies (Berndt & Savin-Williams, 1993; Hartup, 1993). Moreover, children whose friendships are higher in positive features and lower in negative features usually are higher not only in self-esteem but also in social, psychological, and academic adjustment. Illustrative data from a longitudinal study of seventh and eighth graders are shown in Table II. The measures of friendship quality were derived from students' reports on the features of three best friendships. Students whose friendships were higher in positive features and lower in negative features were higher in classroom TABLE II Correlations of the Positive and Negative Features of Multiple Friendships with Measures of Students' Adjustment in the Fall and in the Spring of a School Year Positive features Adjustment measure Self-reported involvement Self-reported disruption Global self-worth

Negative features

Fall

Spring

Fall

.24"* * -.06 .16"*

.24"** -.08 .18 . . . .

- . 11" .29*** .27***

Spring - . 17"* .28*** -.20***

Note: The correlations for the students' classroom involvement and disruption were reported by Berndt and Keefe (1995); those for the students' global self-worth were reported by Keefe and Berndt (1996). *p<.05. **p<.01. ***p<.001.

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involvement and in global self-worth in the fall and the spring of the school year. Students whose friendships were higher in negative features were also higher in disruptive behavior. None of the correlations are strong, however, and the correlations of positive features of friendship with global self-worth are surprisingly weak. Because the study had a longitudinal design, whether high-quality friendships contributed to positive changes in students' involvement, disruption, or self-worth could also be evaluated. Specifically, each of the measures of involvement, disruption, and self-worth in the spring of the year was the criterion in a separate hierarchical regression analysis with the corresponding fall measure as the first predictor. As discussed earlier, this order of entry controls for the continuity in students' characteristics over time. At the next step in the analysis, either the measure of positive features or the measure of negative features was entered. If those measures added significantly to the prediction of the criterion, they were assumed to influence the changes over time in students' characteristics. The results of the regression analyses were both surprising and definitive (Berndt & Keefe, 1995; Keefe & Berndt, 1996). The measure of the positive features of students' friendship was not a significant predictor of the changes during the year in students' global self-worth or, for that matter, involvement and disruption. Indeed, the regression coefficient for positive features as a predictor of changes in global self-worth was .00 (Berndt, 1996). Comparable analyses in two other longitudinal studies also yielded nonsignificant results (Berndt, Hawkins, & Jiao, 1999; Hirsch & DuBois, 1991). Moreover, the three longitudinal studies included different measures of friendship quality, so the null results cannot be attributed to the flaws of a single measure. Given these consistent findings, the hypothesis that supportive friendships enhance children's self-esteem should probably be relegated to the category of a myth rather than a truth about the effects of friendship quality. If the hypothesis about the contributions of positive friendship features to children's self-esteem is rejected, the question about how the quality of children's friendships influences their adjustment must be reconsidered. Because the dominant theories of friendship quality have emphasized the self-esteem hypothesis, they provide little guidance on this question, but research findings suggest two possibilities. First, children who have friendships high in positive features may be more able to form positive relationships with other peers. Kindergarten children in one study who viewed their friends as more helpful and supportive in the middle of the school year gave more positive reports on their classmates' behavior toward them as the year progressed (Ladd, Kochenderfer, & Coleman, 1996). In another study (Bemdt et al., 1999), sixth graders in elementary school who viewed their friendship as having more positive features adjusted better, socially, after the move to seventh grade in junior high school if they kept most of their old friendships after the move. More specifically, classmates judged students as increasing in their sociability

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and leadership after the transition if they had high-quality friendships that were generally stable. Apparently, students who have a few high-quality friendships are able, if they maintain those friendships, to develop better relationships with the rest of their classmates. High-quality friendships seem to provide children with a small social circle that makes them feel they belong in a new school environment. If that circle remains intact, it can easily widen to include more classmates. As the circle widens, the children maintain the distinction between best friends and other classmates, but they increasingly view their other classmates as helpful (Ladd et al., 1996). The other classmates, in turn, increasingly view them as sociable and as socially skilled (Berndt et al., 1999). In this way, success in the small social world of a friendship group contributes to success in the larger social world of a classroom or an entire school. Second, having friendships high in negative features may negatively affect children's social behavior toward other peers and adults. The seventh and eighth graders in one study described earlier (Berndt & Keefe, 1995) increased in self-reported disruptive behavior during a school year if their friendships in the fall had more negative features. This effect suggests that conflicts, dominance struggles, and rivalry among friends spill over to affect students' social behavior toward other classmates and teachers. But even more alarming, the effect of the negative features of friendship was qualified by a significant interaction with positive friendship features. Among students whose friendships were relatively low in positive features, variations in negative friendship features were unrelated to the changes over time in disruptive behavior. These friendships apparently were not very close and so were not very influential. By contrast, among students whose friendships were relatively high in positive features, variations in negative friendship features were strongly related to increases over time in disruptive behavior. In these close and rewarding friendships, students apparently have many opportunities to learn and practice a repertoire of social behaviors with their friends. When that repertoire is heavily weighted toward conflicts, dominance, and rivalry, the consequences for the students' social interactions with other people are especially negative. Rather than being confined to their interactions with their friends, their negative social repertoire spills over to their interactions with other peers and with adults. Given the practical importance of students' disruptive behavior, more research on how this behavior is influenced by children's relationships with their friends is greatly needed. This statement, in more general form, becomes the next recommendation for future research: Research recommendation:Examinethe separate and combinedeffects of positive and negative friendship features. As noted earlier, few studies have included independent measures of positive friendship features and negative friendship features. Even fewer have examined

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the possible interactions between the two. Of course, strong conclusions cannot be drawn from the single significant interaction reported by Berndt and Keefe (1995). Nevertheless, interactions of this type cannot be discovered unless researchers assess both positive and negative friendship features, and analyze their separate and combined effects, when studying the influences of friendship quality.

IV. Influences of Friends' Characteristics in Friendships Differing in Quality Most researchers interested in the first pathway of friends' influence--the influence of friends' characteristicsmhave said little or nothing about the quality of the relationships among those friends. Conversely, most researchers interested in the second pathway of friends' influence the influence of friendship qualitywhave said nothing about the characteristics of the children who are friends. The separation between the two lines of research reflects their focus on different questions and their origins in different theoretical traditions. One consequence of the separation is that few researchers have tried to test hypotheses about interactions between the two influence pathways. That is, few researchers have tried to determine whether the influences of friends with certain characteristics are different when those friendships differ in quality. As a result, no strong assumptions about such interactions are found in the literature. In one sense, that is good news, because it implies that no myths about these interactions are being presented as truths in the absence of supporting data. In another sense, the lack of attention to interactions between influence pathways is bad news, because it means that stating what is true about those interactions is not yet possible. Some hypotheses about interactions between the influence pathways are part of prominent theories of social influence and social behavior. The central hypothesis in one theory of delinquent behavior is that adolescents who spend more time with delinquent friends are more likely to commit delinquent acts. A secondary hypothesis is that the negative effect of association with delinquent friends is stronger when adolescents have more positive relationships with those friends (Agnew, 1991). In other words, the influence of the friends' delinquent behavior is magnified when their friendships are higher in quality. Similarly, social learning theory (Bandura, 1977) includes the hypothesis that children learn more from observing a model's behavior if they have a positive relationship with the model. Therefore, observational learning from a friend should be enhanced when children view that friendship as having more positive features. A related hypothesis drawn from sociological theories of influence is that friends are more influential when their friendships are based on greater trust in one another (Hallinan & Williams, 1990). Trust is closely linked to intimacy, because children are only willing to share intimate information about themselves with close friends

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whom they trust. Therefore, this theory also implies that the influences of friends and friendships interact. Stated more generally, all of these theories imply that the influence of friends is magnified when those friendships are higher in quality. For ease of reference, an interaction between the two influence pathways that takes this specific form can be defined as supporting the magnification hypothesis. The magnification hypothesis may seem like common sense, but it is controversial and has been challenged. In particular, theories that focus on social support in close relationships typically include the assumption that support from friends is beneficial regardless of the characteristics of those friends (Berndt, 1989). Similarly, one prominent theory of juvenile delinquency includes the assumption that adolescents who are more strongly attached to a group of friends will be less delinquent, regardless of how delinquent those friends are (Hirschi, 1971). For this reason, evidence for the magnification hypothesis would bolster some theories of social influence while lessening support for others. Besides their theoretical significance, questions about the influences of friends' characteristics when friendships differ in quality are of great practical significance. Consider a plan for an intervention with adolescents who are at high risk of dropping out of high school. Suppose that the intervention includes enjoyable activities for small groups of these adolescents, and one result of these activities is that the adolescents in each group become good friends with one another. Are those good friendships desirable or undesirable? The findings of two such interventions suggest that those friendships must be viewed as problematic at best (Catterall, 1987; Hymel, Comfort, SchonertReichl, & McDougall, 1996). In both interventions, the dropout rate was higher for participating students than for students in a control condition. Further analyses suggested that the interventions brought together students who were alienated from school and the groups' activities did not change their attitudes substantially or permanently. After the interventions ended, students returned to the regular school program, and some of them decided to drop out of school. When they reported their decisions to their new friends, most of them also decided to drop out. Thus, the interventions increased the influence of dropout-prone students on other students by facilitating the formation of friendships among those students. Other interventions have been implemented with the goal of reducing the antisocial and delinquent behavior of high-risk adolescents (Dishion et al., 1999). Some of these adolescents were placed in groups and given opportunities for frequent interactions with one another, either in brief training sessions or during a summer camp. Unfortunately, adolescents in these groups displayed a higher level of antisocial and delinquent behavior after the interventions than did adolescents in control groups. Again, the formation of friendships among adolescents who already were on a deviant developmental trajectory seemed to be the best explanation for the harmful effects of interactions that were intended to be helpful.

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The findings of these interventions could be interpreted as providing support for the magnification hypothesis, but that conclusion would be premature. Some interventions that have brought together children or adolescents who have problems in social or psychological adjustment have had positive outcomes (see Dishion et al., 1999). That is, these interventions have been successful in increasing the participants' social or psychological adjustment, even though they also created opportunities for forming friendships with other participants who had problems in adjustment. Moreover, none of the intervention programs really tested the magnification hypothesis because none included measures of friendship quality. Only a few studies have included measures of both friendship quality and friends' characteristics, and those few have yielded mixed results. In one study (Agnew, 1991), adolescents whose friends engaged in more serious forms of delinquent behavior increased over time in seriously delinquent behavior if they were closely attached to those friends. However, comparable results were not found for a measure of minor delinquent behaviors. In studies with more standard measures of friendship quality, evidence for the hypothesis is even weaker. Berndt and Keefe (1995) did not discuss interactions between the influences of friends' characteristics and friendship quality because very few were significant and those could be attributed to chance. Berndt, Hawkins, and Jiao (1999) reported one interaction that might have supported the magnification hypothesis, but tests done to decompose that interaction were inconclusive. Poulin, Dishion, and Haas (1999) also reported that interactions between measures of friends' characteristics and friendship quality were nonsignificant. Nevertheless, the evidence that peer-oriented intervention programs sometimes have negative effects on adolescents' behavior cannot be ignored. At a minimum, further research is needed to clarify the explanation for those effects. More generally, additional research on the magnification hypothesis would be valuable. Stated more formally: Research recommendation: Interactionsbetween the influences of friends' characteristics and friendship quality shouldbe systematicallyexplored. Interactions between friends' characteristics and friendship quality need not take the form specified by the magnification hypothesis. In the study described earlier of the transition to junior high school (Berndt et al., 1999), an interaction between friends' characteristics and friendship quality was found for a measure of students' shyness and social withdrawal as rated by classmates. When classmates viewed a student's friends in sixth grade as high in shyness and withdrawal, the student's shyness and withdrawal increased significantly after the transition, but only if those friendships were low or average in quality. Students who had highquality friendships in sixth grade did not become more shy and withdrawn over time, even if their sixth-grade friends were shy and withdrawn.

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This result is paradoxical because a straightforward reading of social learning theory would suggest exactly the opposite. The students' classmates judged their shyness and withdrawal from their pattern of social behavior. Friends could observe this pattern of social behavior as easily as other classmates could. Consistent with the magnification hypothesis, greater imitation of the friends' pattern of social behavior might have been expected when students had higher quality relationships with those friends. The results for shyness and withdrawal can be more plausibly explained by referring to hypotheses about the protective effects of supportive relationships (Berndt, 1989). The most important benefit of good friendships may be to make students feel comfortable and secure in the school environment. Students may receive this benefit even if those friends are shy and withdrawn themselves. Therefore, the students who had good friendships with shy, withdrawn friends felt no inclination to adopt their friends' profile of social behavior. By contrast, students who did not have good friendships may have felt lost, socially, in the new school. Lacking the sense of security that good friendships can provide, those students became more shy and withdrawn. This explanation is obviously speculative but nonetheless important. It illustrates that future studies of the influences of friends and friendships have the potential to challenge, refine, and extend major theories of social development. It also illustrates the limitations of research that provides data only on the influences of friends' characteristics or only on the influences of friendship quality. Conclusions about either one will be misleading if the two types of influences interact.

V. Conclusions and Implications Understanding of the influences of friends and friendships has increased dramatically since theorists such as Sullivan (1953) and researchers such as Bronfenbrenner (1967, 1970) brought attention to these phenomena. Research has exposed many myths and revealed some truths about these influences. The most important of those myths and truths deserve to be restated because they have broader implications for the field of child development. Those implications relate to theories, research methods, and intriguing parallels between friendship research on parentchild relationships. The influences of friends and friendships can be understood best by distinguishing between two pathways of friends' influence (Berndt, 1992, 1999b). One pathway is through the attitudes, behavior, and other characteristics of friends. The widespread belief that friends' influence through this pathway is predominantly negative, especially in adolescence, has been shown conclusively to be a myth. Friends can have either a positive influence or a negative influence on children and adolescents, depending on whether the friends' own characteristics are positive or

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negative. For example, children with friends who are disruptive in school are likely over time to become more disruptive themselves, but children with friends who are well behaved in school are likely to behave better over time. Stated more generally, friends' influence in nearly all cases makes children's attitudes and behaviors more similar to the attitudes and behaviors of their friends. One reason for the persistence of the myth of negative friends' influence is that research in this area has been theoretically weak. Indeed, much of the research has been completely atheoretical, guided by popular beliefs about adolescents or by mere extensions of previous studies. Several theories of social influence have been the foundation for experimental studies of peers' contributions to children's socialization (Hartup, 1970, 1983), but those theories have rarely been the foundation for research on friends' influence. For further advances in understanding of friends' influence, research must be theoretically grounded and designed to contribute to the testing and refinement of general theories of social influence. Yet even in its current state, research on the influences of friends' characteristics raises intriguing issues for all child-development researchers. One issue deals with a dramatic contrast between the usual frameworks for investigating friends' influence and the most prominent theories of parents' influence. Theories of parents' influence focus almost exclusively on parent-child relationships and on parents' direct and indirect training of their children (e.g., Maccoby & Martin, 1983; Parke & Buriel, 1998). These general theories rarely emphasize how parents' characteristics might influence their children's behavior and development. For example, when trying to explain students' disruptive behavior, parenting researchers would likely focus on how parents interact with and discipline their children, how parents instruct their children about appropriate social behavior, and how parents manage their children's social interactions with peers (Parke & Buriel, 1998). But what about the parents' own disruptive behavior toward other people, either in the home or in other settings where their children can observe their behavior? The idea that children take their parents as models for their own behavior is not novel (see, e.g., Bandura & Waiters, 1959), but this idea has not received much attention over the years. One implication of the research of friends' influence is that revising theories of parenting to include hypotheses about the model that parents provide for their children, even when not interacting with their children, would be worthwhile. Research on the influence of friends' characteristics has also shown conclusively that social pressure is not the primary process by which friends influence children. Children who are close friends rarely put pressure on one another, and they are often ineffective in changing one another's decisions or behavior when they do so. More important for influence among friends are social reinforcement, observational learning, and rational discussion based on information exchange. The myth of strong peer pressure has endured partly because researchers have often used methods that assume its existence. Starting with Bronfenbrenner (1967),

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researchers have asked children to respond to hypothetical dilemmas in which friends or other peers were said to pressure them to do something they did not want to do. In other studies (e.g., Clasen & Brown, 1985), researchers directly asked children how much pressure to engage in certain behaviors or activities they received from friends. To obtain a more accurate picture of influence processes among friends, we recommend that researchers conduct systematic observations of friends' interactions in natural settings. We also recommend that researchers conduct experimental studies of friends' discussions, because that method allows researchers to examine the processes and outcomes of friends' influence simultaneously. The data already available on influence processes among friends are surprisingly comparable to data obtained in research on parental influence (Maccoby & Martin, 1983). For example, just as children are often ineffective when they try to change their friends' behavior by applying coercive pressure, parents are often ineffective when they try to change their children's behavior by using power-assertive discipline. Similarly, the effectiveness of rational arguments in changing children's opinions during discussions with friends is analogous to the effectiveness of parenting practices that emphasize reasoning or inductive discipline. These parallels between the techniques that friends and parents use when attempting to change children's behavior are intriguing and suggest many questions for future research. One obvious question is whether influence techniques are learned at home. That is, do children who are trying to influence their friends mainly use the influence techniques that their parents most often adopted when trying to influence them? In addition, research has shown that the power that friends have to influence children's attitudes and behaviors has often been exaggerated. One reason for the persistence of the myth of powerful friends' influence is that many researchers have estimated friends' influence from the similarity at one time between children's characteristics and their friends' characteristics. These researchers ignored the reality that friends' similarity on many characteristics is due partly to children's selection of friends to whom they are already similar. An analogous problem has existed for decades in research on parents' influence. That is, many researchers have tried to determine the influence of parents on their children's behavior and adjustment by correlating measures of parents' practices with measures of their children's characteristics. The problem with this method is not that of selection----children do not choose their parents and parents do not exactly choose their children--but measures of parents' practices and of children's characteristics could be correlated because of the genetic similarity between parents and their children. Prominent critics of parenting research have argued that such parent-child correlations can be better explained by the transmission of genes from parent to child than by social influence of parents on children (e.g., Harris, 1998; Scarr, 1992).

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In response to these criticisms, parenting researchers have emphasized the evidence for parental influence from studies that did not have correlational designs (Collins, Maccoby, Steinberg, Hetherington, & Bornstein, 2000). Specifically, these researchers have pointed to evidence on parents' influence from studies with longitudinal or experimental designs. We also argued that those designs have provided the best evidence on how and how much friends' influence children's behavior. These parallels illustrate that researchers who study friends' influence and parents' influence are partners in the debate against researchers who assert that children's development is not greatly influenced by any of the people with whom they have close personal relationships. A shift in focus to the second influence pathway, the influence of the quality of children's friendships, reveals a different set of myths and truths. The best-known hypothesis about friendship quality is that it enhances children's self-esteem. In several longitudinal studies, however, having high-quality friendships was not associated with improvements over time in children's self-esteem. Findings from those and other studies did suggest that having friendships high in positive features helps children form better relationships with other peers and so enhances their success in the peer social world. Friendships high in negative features (e.g., conflicts, dominance, and unpleasant rivalry) appear to increase children's negative behaviors toward other peers and teachers. This negative effect is most evident when friendships are also high in positive features. Apparently, practicing a repertoire of negative social behaviors during interactions with close friends has a negative influence on children's interactions with other people as well. These conclusions must be considered tentative because only a few studies of the effects of friendship quality have been reported. The conclusions should also be considered tentative because they cannot be linked either to a specific theory of the effects of friendships or to a general theory of the effects of close relationships. Such a general theory might be formulated by linking data on the effects of friendship quality to data on the effects of other relationships, particularly relationships with parents. For example, effects comparable to the spillover from negative behavior with friends to negative behavior toward other people have been shown to occur in parent-child relationships. In particular, children who regularly practice a repertoire of coercive behavior when interacting with parents increasingly display coercive behavior toward classmates and teachers at school as well (Patterson, Reid, & Dishion, 1992). Moreover, the learning of this coercive repertoire is governed by principles of reinforcement that may be applicable to all close relationships. In other respects, the influences of parents on their children may be qualitatively different from the influences of friends and friendships on children. In working toward a general theory of the influence of close relationships, the goal would not be to prove that all types of relationships have the same types of effects. Rather,

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the goal would be to increase understanding of each important type of relationship by describing both how it is similar and how it is different from other types. Finally, a general theory of the influence of close relationships must account for interactions between different pathways of influence. These interactions may take various forms, but the greatest attention has focused thus far on the hypothesis that the influence of a friend's characteristics is magnified when that friendship is higher in quality. This magnification hypothesis is included in several but not all theories of social influence. The hypothesis is practically significant because it implies that interventions to improve the quality of the friendships among children with negative characteristics (i.e., children high in antisocial behavior) will have harmful rather than beneficial effects on the children's behavior and adjustment. In theory, the magnification hypothesis is as applicable to parent-child relationships as to friendships. Many parenting researchers would readily accept the positive version of the hypothesis. That is, they assume that parents' efforts to model positive social behaviors are most effective when the parents also are warm and supportive of their children (Maccoby & Martin, 1983). But what about the negative version of the hypothesis? What about parents whose behavior is as socially undesirable as that of delinquent adolescents? Do parents who engage in violent or criminal behaviors have an especially negative influence on their children's behavior when they are also warm and supportive of their children? No answer to these questions can be given until much more research is done. Even as applied to friends and friendships, evidence relevant to the magnification hypothesis is limited and inconsistent. Researchers who have examined the influence of friends' characteristics have rarely assessed the quality of those friendships, and researchers who have examined the influence of friendship quality have rarely assessed the characteristics of the friends. With respect to parent-child relationships, the evidence on the magnification hypothesis is even more limited. Nonetheless, the significance of the hypothesis is extremely clear. If the hypothesis is disconfirmed by future research either on friendships or on parent-child relationships, major theories of social influence will need to be substantially revised. If the hypothesis is confirmed for either type of relationship, many social programs will need to be reconsidered. If the negative influences of friends are magnified when those friendships are high in quality, programs that bring together high-risk youth with the aims of preventing negative outcomes (e.g., delinquency, school dropout) will need to be reevaluated (Dishion et al., 1999). If negative effects of parents' characteristics are magnified when parent-child relationships are warmer and more supportive, programs of parent training may need to focus not only on improving parents' techniques for interacting with their children but also on ensuring that the parents are models of positive social behavior for their children. In sum, research on the influences of friends and friendships has begun to yield important information about how friends affect children's attitudes and behaviors. But this research has broader implications and intriguing connections with research

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on other close relationships, particularly those between parents and children. These connections may provide the starting points for a general theory of the influence of close relationships on children's development. Such a theory could, in turn, provide a basis for interventions designed to enhance children's development.

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Hartup, W. W. (1970). Peer interaction and social organization. In E H. Mussen (Ed.), Carmichael's manual of child psychology (3rd ed., pp. 361-456). New York: Wiley. Hartup, W. W. (1983). Peer relations. In E H. Mussen (Series Ed.) & E. M. Hetherington (Vol. Ed.), Handbook of child psychology: Vol. 4. Socialization, personality, and social development (4th ed., pp. 103-196). New York: Wiley. Hartup, W. W. (1993). Adolescents and their friends. In B. Laursen (Ed.), New directions for child development: Close friendships in adolescence (pp. 3-22). San Francisco: Jossey-Bass. Hartup, W. W. (1996). The company they keep: Friendships and their developmental significance. Child Development, 67, 1-13. Hirsch, B. J., & DuBois, D. L. (1991). Self-esteem in early adolescence: The identification and prediction of contrasting longitudinal trajectories. Journal of Youth and Adolescence, 20, 53-72. Hirschi, T. (1971). Causes of delinquency. Los Angeles: University of California Press. Howes, C. (1996). The earliest friendships. In W. M. Bukowski, A. E Newcomb, and W. W. Hartup (Eds.), The company they keep: Friendship in childhood and adolescence (pp. 66-86). New York: Cambridge University Press. Hymel, S., Comfort, C., Schonert-Reichl, K., & McDougall, E (1996). Academic failure and school dropout: The influence of peers. In J. Juvonen & K. R. Wentzel (Eds.), Social motivation: Understanding children's school adjustment (pp. 313-345). New York: Cambridge University Press. Ide, J. K., Parkerson, J., Haertel, G. D., & Walberg, H. J. (1981). Peer group influence on educational outcomes. A quantitative synthesis. Journal of Educational Psychology, 73, 472-484. Isenberg, D. J. (1986). Group polarization: A critical review and meta-analysis. Journal of Personality and Social Psychology, 50, 1141-1151. Janis, I. L. (1982). Groupthink: Psychological studies of policy decisions and fiascoes. Boston: Houghton Mifflin. Kandel, D. B. (1978). Homophily, selection, and socialization in adolescent friendships. American Journal of Sociology, 84, 427-436. Kandel, D. B. (1996). The parental and peer contexts of adolescent deviance: An algebra of interpersonal influences. Journal of Drug Issues, 26, 289-315. Keefe, K. (1994). Perceptions of normative social pressure and attitudes toward alcohol use: Changes during adolescence. Journal of Studies in Alcohol, 55, 46-54. Keefe, K., & Berndt, T. J. (1996). Relations of friendship quality to self-esteem in early adolescence. Journal of Early Adolescence, 16, 110-129. Keenan, K., Loeber, R., Zhang, Q., Stouthamer-Loeber, M., & Van Kammen, W. B. (1995). The influence of deviant peers on the development of boys' disruptive and delinquent behavior: A temporal analysis. Development and Psychopathology, 7, 715-726. Ladd, G. W., Kochenderfer, B. J., & Coleman, C. C. (1996). Friendship quality as a predictor of young children's early school adjustment. Child Development, 67, 1103-1118. Maccoby, E. E., & Martin, J. M. (1983). Socialization in the context of the family: Parent-child interaction. In P. H. Mussen (Series Ed.) & E. M. Hetherington (Vol. Ed.), Handbook of child psychology: Vol. 4. Socialization, personality, and social development (4th ed., pp. 1-101). New York: Wiley. McPherson, M., Smith-Lovin, L., & Cook, J. M. (2001). Birds of a feather: Homophily in social networks. Annual Review of Sociology, 27, 415-444. Parke, R. D., & Buriel, R. (1998). Socialization in the family: Ethnic and ecological perspectives. In W. Damon (Editor-in-chief), N. Eisenberg (Vol. Ed.), Handbook of child psychology (5th ed.). Vol. 3. Social, emotional, and personality development (pp. 463-552). New York: Wiley. Patterson, G. R., Reid, J. B., & Dishion, T. J. (1992). A social learning approach. Vol. 4. Antisocial boys. Eugene, OR: Castalia Press. Piaget, J. (1932/1965). The moral judgment ofthe child. New York: Free Press. (Original work published 1932.)

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Author Index

A

Asarnow, R., 233, 272 Asendorpf, J. B., 210, 219, 221 Asher, S. R., 79, 99 Aslin, R. N., 166, 182 Aspinwall, L. G., 196, 221 Atkinson, L., 251,258, 263, 274 Atran, S., 76, 90 Avolio, A. M., 3, 12, 21, 25, 26, 31, 36, 38 Ayduk, O., 212, 221

Aboud, E E., 289, 307 Achenbach, T. M., 214, 220 Acredolo, L. P., 27, 36 Adamson, L. B., 247, 268 Adolph, K. E., 3, 4, 6, 9, 12, 17, 21, 25, 26, 27, 28, 29, 30, 31, 36, 37, 38, 39 Agnew, R., 299, 301,307 Ahadi, S. A., 121, 151, 197, 204, 206, 207, 212, 220, 227 Ahlgren, A., 79, 89 Ainsworth, M. S., 233, 234, 236, 237, 239, 240, 241,244, 260, 262, 263 Alansky, J. A., 243, 244, 268 Alatalo, R. V., 61, 95 Albert, G., 120, 123, 149 Alberts, S. C., 50, 51, 62, 90 Albon, S. D., 59, 92 Alekseev, V., 50, 90 Alexander, R. D., 42, 44, 45, 51, 63, 64, 65, 70, 86, 87, 88, 90 Allen, D., 253, 263 Allen, J. R., 169, 182 Allen, R., 246, 250, 274 Allman, J., 58, 59, 64, 69, 73, 74, 87, 90 Altmann, J., 50, 51, 62, 90 Altshuler, J. L., 202, 203, 220 Alvarez, H., 64, 70, 95 Amaral, D. G., 109, 152 American Psychiatric Association, 252, 260, 263 Anders, T., 247, 263 Andersson, M., 41, 48, 51, 52, 53, 54, 56, 57, 87, 90, 98 Andrews, D. W., 308 Ansley, L., 283, 307 Anson, A. R., 283,284, 290, 308 Archer, J., 42, 90 Arcus, D. M., 197, 225 Aronson, E., 25, 38 Arterberry, M., 24, 40, 159, 183

B

Baak, K., 240, 269 Bachevalier, J., 109, 146 Backer, L. F., 175, 185 Bacon, A. L., 253, 263 Bahrick, L. E., 154, 155, 156, 157, 158, 159, 164, 166, 167, 168, 170, 171,172, 173, 174, 177, 179, 180, 181,183, 184, 186 Bai, D., 4, 25, 27, 37 Baillargeon, R., 148 Bakeman, R., 246, 247, 265, 268 Baker, N., 207, 226 Baker, R. R., 72, 73, 90 Bakermans-Kranenburg, M. J., 239, 254, 259, 272,274 Baker-Ward, L., 120, 123, 139, 149 Baldwin, D. A., 199, 226 Band, E., 205,221 Bandura, A., 284, 299, 303, 307 Bangston, S. K., 111,114, 115, 117, 118, 138, 147 Banker, H., 161, 184 Bardy, B. G., 154, 155, 186 Barnard, K. E., 247, 248, 263 Barnett, D., 190, 222, 235, 236, 251,252, 260, 263, 267 Barnett, M. P., 110, 146 Barocas, R., 232, 272 Baron-Cohen, S., 64, 90 Barr, R., 111,114, 146 311

312

Author Index

Barrett, K. C., 24, 28, 29, 37, 193, 194, 199, 221 Barrett, T., 31, 36 Barth, D. S., 160, 181 Bartini, M., 78, 86, 98 Bartusch, D. J., 208, 209, 229 Basham, R. B., 246, 247, 248, 266 Bates, E., 119, 148 Bates, J. E., 71, 72, 93, 121,122, 151,197, 208, 212, 214, 226, 227 Bauer, P. J., 105, 107, 108, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 152 Bauman, K. E., 287, 308 Baumgartner, P., 253,269 Bausano, M., 24, 26, 39 Beck, S. P., 74, 90 Beckwith, L., 233,240, 241,247, 248, 255, 256, 257, 264, 267, 271,272, 273 Bee, H. L., 247, 248, 263 Beebe, B., 157, 183, 238, 240, 269 Beer, J., 124, 148 Beerli, P., 44, 96 Beike, D. O. R., 144, 149 Bell, R. Q., 231,264 Belle, D., 79, 90 Bellis, M. A., 72, 73, 90 Belsky, J., 71, 90, 234, 238, 240, 243, 244, 264, 268 Bennett, K. L., 69, 93 Bento, S., 242, 271 Berenbaum, S. A., 79, 82, 84, 90, 96, 98 Berg, C. A., 203, 221 Berger, S. E., 26, 36 Bergman, A., 237, 270 Berkeley, G., 154, 181 B erman, P. W., 81, 90 Berndt, T. J., 276, 278, 279, 282, 283,284, 285, 286, 288, 289, 292, 293, 294, 295,296, 297, 298, 299, 300, 301,302, 307, 309 Bernstein, N., 25, 33, 36 Bernstein, V. J., 257, 268 Berntson, G. G., 198, 221 Bernzweig, J., 201, 221 Berrigan, D., 44, 96 Bertenthal, B. I., 4, 24, 25, 27, 28, 29, 37 Biancaniello, R., 6, 30, 31, 37 Biederman, J., 214, 221,227

Binns, K. E., 160, 161,187 Birch, H., 154, 181 Biringen, Z., 237, 238, 239, 240, 241,264 Bishop, J. H., 286, 308 Bittinger, K. A., 128, 146 B iu, O., 29, 36 Bjorklund, D. E, 42, 45, 63, 76, 90, 94, 116, 151 Blackwell, J., 235,236, 251,252, 254, 263 Blake, R., 177, 186 Blatt, S. J., 80, 96 Blehar, M. C., 233,234, 236, 237, 239, 240, 241,244, 260, 262, 263 Block, J., 195, 197, 210, 215,216, 221 Block, J. H., 195, 197, 210, 215, 216, 221 Blomberg, S. P., 68, 69, 70, 96 Bloom, M., 239, 272 Blumberg, S. L., 82, 94 Blurton Jones, N. G., 64, 70, 85, 86, 90, 95 Boesch, C., 64, 70, 95 Bogin, B., 42, 63, 64, 91 Bolduc, E. A., 214, 221 Bolduc-Murphy, E. A., 214, 227 Bolzern, A. M., 48, 56, 99 Bond, L., 80, 91 Borgerhoff Mulder, M., 70, 72, 91 Borgia, G., 60, 61, 88, 91, 92 Bornstein, M. H., 202, 203,220, 305, 308 Borton, R. W., 157, 184 Bossi, T., 62, 93 Bost, K. K., 243,244, 274 Boster, J. S., 72, 73, 74, 94 Botein, S., 258, 270 Bothamley, J., 191, 221 Bouchard, T. J., Jr., 84, 98 Bovenhuis, H., 70, 92 Bowlby, J., 233,234, 239, 256, 264 Bowman, J. E., 81, 96 Braafladt, N., 203, 224 Brachfeld, S., 246, 247, 248, 264, 268 Brainard, M. S., 160, 161, 183 Bramhall, M. A., 142, 152 Braungart, J. M., 200, 228 Brazelton, T. B., 238, 264 Brelsford, K. A., 253, 269 Breniere, Y., 4, 37 Brenner, E. M., 198, 221 Bretherton, I., 192, 200, 221,237, 240, 241, 264 Brett, B., 160, 181 Bridges, E A., 251,265

Author Index

Bridges, L. J., 199, 200, 202, 204, 207, 208, 221,224 Briggs-Gowan, M. J., 193, 204, 212, 222 Bill, B., 4, 37 Brill, N., 257, 271 Brinich, P. M., 249, 265 Brodersen, L., 199, 224 Bronfenbrenner, U., 277, 283, 291,302, 303, 308 Bronfman, E., 239, 270 Brooks, R., 62, 91 Brooks-Gunn, J., 251,265 Brown, A. L., 116, 148 Brown, B. B., 280, 304, 308 Brown, E., 168, 173, 180, 186, 266 Brown, G. M., 109, 149 Brown, J., 205,223,248, 270 Brown, J. H., 44, 100 Brown, J. V., 246, 247, 265 Bruce, J., 145, 150 Bruce, K., 11, 29, 37 Bryan, W. L., 33, 37 Bryga, H., 48, 98, 99 Buhrmester, D., 277, 308 Buitelaar, J. K., 254, 274 Bukowski, W. M., 78, 97 Burch, M. M., 127, 128, 129, 130, 131, 132, 133, 134, 138, 141,143, 144, 146, 147 Buriel, R., 303, 309 Burnstein, E., 285, 310 Burraston, B., 284, 286, 308 Busnel, M. C., 175, 182 Buss, A. H., 268 Buss, D. M., 54, 66, 67, 69, 71, 75, 91 Butler, C. M., 235, 260, 263 Butterworth, G., 158, 181 Byers, J. A., 45, 91 Byrne, R., 82, 83, 91

Cabeza, R., 109, 147 Cacioppo, J. T., 198, 221 Calamari, G., 272 Calkins, S. D., 193, 221 Calvert, G. A., 159, 160, 162, 182 Campbell, A., 66, 91 Campbell, K., 239, 271

313

Campbell, S. B., 257, 258, 259, 265 Campos, J. J., 11, 24, 27, 28, 29, 37, 191,193, 194, 195, 199, 201, 221,222 Campos, R. G., 191,193, 194, 195, 221,222 Capaldi, D. M., 205, 206, 222 Caparulo, B. K., 282, 285, 286, 307 Capps, L., 235, 253,254, 265, 272 Capuzzi, C., 265 Carey, W. B., 123, 147 Carlile, S., 159, 161,183 Carlin, J. B., 80, 91 Carlo, G., 212, 223 Carlsen, R. M., 161, 182 Carlson, E. A., 234, 235, 236, 265, 273,274 Carmichael, H., 249, 270 Caron, A. J., 172, 182 Caron, R. F., 172, 182 Carranza, J., 49, 59, 91 Carstensen, L. L., 196, 222 Carter, A. S., 193, 204, 212, 222, 259, 265 Carver, L. J., 108, 110, 111,114, 117, 118, 126, 127, 137, 138, 140, 147, 148 Cary, M. S., 120, 139, 146 Casas, J. E, 67, 69, 79, 80, 86, 89, 92 Casey, B. J., 209, 222 Caspi, A., 207, 208, 209, 210, 211, 212, 222, 225, 226, 227, 229, 293,308 Cassidy, D. J., 116, 148 Cassidy, J., 193, 222, 237, 238, 239, 242, 265, 270 Catterall, J. S., 300, 308 Cavalli-Sforza, L. L., 66, 99 Chagnon, N. A., 65, 66, 70, 85, 91 Chaloff, J., 214, 227 Chan, M., 6, 30, 31, 37 Chan, M. Y., 6, 30, 31, 37 Chang, P., 248, 271 Chao, C.-C., 79, 96 Chapman, M., 258, 271 Charlesworth, W. R., 77, 91 Charness, N., 33, 37 Charnov, E. L., 41, 44, 46, 64, 70, 91, 95 Chazan-Cohen, R., 259, 265 Cheatham, C. L., 120, 139, 146 Chen, R., 253, 269 Chen, Z., 83, 91 Chess, S., 121,152, 244, 265, 268 Chin, J., 9, 38 Chisholm, G., 240, 269 Chisholm, J. S., 71, 72, 73, 91

314

Author Index

Chisholm, V. C., 235, 236, 251,252, 254, 263 Chugani, H. T., 123, 148 Cicchetti, D., 190, 204, 222, 228, 236, 247, 251,252, 258, 263, 265, 267, 273 Clark, J. E., 25, 39 Clark, S., 160, 183 Clasen, D. R., 280, 304, 308 Clearfield, M. W., 3, 36 Clinton, W. L., 57, 58, 92 Clutton-Brock, T. H., 48, 49, 51, 52, 53, 57, 58, 59, 64, 92, 95 Cohen, J. M., 287, 308 Cohen, M. C., 273 Cohen, N. J., 109, 148 Cohen, S. E., 233, 240, 241,247, 248, 264, 272 Cohn, D. A., 259, 271 Cohn, J. F., 238, 240, 257, 258, 259, 265,266 Coie, J. D., 206, 207, 225,229 Colder, C. R., 213,222 Coldren, J. T., 169, 182 Cole, N. S., 83, 101 Cole, P. M., 190, 197, 202, 208, 222 Coleman, C. C., 297, 298, 309 Coleman, S. W., 60, 61, 91 Collaer, M. L., 79, 82, 92 Collins, W. A., 305, 308 Collis, K., 60, 88, 92 Colombo, J., 169, 182 Columbus, R. F., 178, 186 Comfort, C., 300, 309 Compas, B. E., 206, 222 Compos, J. J., 193, 194, 199, 228 Connell, D., 235, 258, 270 Connell, J. P., 200, 202, 207, 208, 221,224 Connelly, A., 109, 152 Conner, J. K., 44, 92, 206, 222 Contreras, J., 193,204, 222 Cook, J. M., 280, 286, 309 Cook, T. D., 283, 284, 290, 308 Coolbear, J., 258, 263 Cooper, L., 72, 94 Cooper, P. E, 246, 249, 268 Cooper, R. P., 166, 182 Cornsweet, T. N., 12, 37 Corter, C., 248, 268 Courage, M. L., 142, 149 Cowan, C. P., 259, 271 Cowan, P. A., 259, 271 Coy, K., 208, 212, 225 Craik, F. I. M., 109, 147, 149

Cranier-Deferre, C., 175, 182 Crawford, J. W., 247, 266 Crawley, S. B., 237, 239, 266 Creasey, G. L., 248, 269 Crenshaw, M. C., 120, 123, 149 Crick, N. R., 67, 69, 79, 80, 86, 89, 92 Crittenden, P. M., 235, 237, 260, 266 Crnic, K. A., 246, 247, 248, 266 Crockenberg, S. B., 245,259, 266 Cross, T. G., 249, 250, 271 Crowell, J., 274 Crown, C. L., 157, 183,238, 240, 269 Culver, C., 191, 192, 193, 226 Cumberland, A., 207, 208, 210, 213, 214, 216, 217, 218, 222, 223, 228 Cummings, E. M., 258, 266, 271 Cunningham, M. R., 71, 92 Cushing, G., 257, 269

Dabbs, J. M., 74, 75, 92 Dabbs, M. G., 74, 75, 92 Dale, E S., 119, 148 Daly, B. P., 114, 117, 118, 119, 147 Daly, M., 69, 71, 72, 73, 74, 87, 92, 101 Damon, W., 284, 308 Darwin, C., 41, 42, 43, 48, 51, 53, 54, 55, 57, 59, 60, 87, 92 Davidson, R. R., 190, 192, 223 Davies, N. B., 52, 96 Davies, P. T., 266 Dawkins, R., 52, 92 Dawson, G., 253, 266, 271 Day, R. H., 24, 25, 39 Dean, J., 235, 271 de Bruin, J. P., 70, 92 DeCasper, A. J., 175, 182 Decker, S. A., 72, 74, 75, 93 DeFries, J. C., 44, 98 de Gaudemar, B., 47, 92 Degirmencioglu, S. D., 292, 293, 310 de Haan, M., 120, 145, 147, 148 DeLoache, J. S., 116, 148 Dempsey, J., 248, 267 DeMulder, E. K., 258, 259, 266 Denham, S. A., 205, 222 Dennig, M. D., 203, 227 Denny, M. A., 6, 27, 28, 36

Author Index

Derryberry, D., 122, 123, 148, 151,195, 197, 198, 218, 222, 243,272 DeSoto, M. C., 72, 94 de Waal, F. B. M., 78, 80, 92 De Wolff, M., 237, 242, 266 Di, S., 160, 181 Diamond, R. M., 161, 183 Dickens, S., 235, 236, 251,252, 254, 263 Dickstein, S., 258, 272 Diener, M. L., 193, 204, 208, 222 DiGirolamo, G. J., 198, 220, 227 DiPietro, J. A., 78, 88, 92 Dishion, T. J., 284, 286, 300, 301,305, 306, 308, 309, 310 Dittami, J., 62, 100 DiVitto, B., 232, 247, 248, 249, 251,266, 268 Dixson, A. E, 62, 93 Dobkin, P. L., 290, 310 Dodge, K. A., 71, 72, 93 Donovan, C., 247, 267 Dorland, M., 70, 92 Doussard-Roosevelt, J. A., 209, 227 Dow, G. A., 128, 146 Dowden, A., 111, 114, 146 Downey, G., 212, 221 Draper, P., 71, 72, 90, 93 Dropik, P., 107, 108, 111,112, 113, 114, 115, 117, 118, 119, 121,124, 128, 130, 138, 139, 144, 147 DuBois, D. L., 297, 309 Dudycha, G. J., 105, 148 Dudycha, M. M., 105, 148 Duffy, D. L., 68, 69, 70, 96 Dukette, D., 122, 150 Dunbar, R. I. M., 53, 59, 69, 74, 93, 95 Dunn, J., 205, 222, 223 Dywan, J., 110, 148 Dzur, C., 77, 91

Eagly, A. H., 89, 93 Eals, M., 83, 99 Earls, E, 197, 210, 212, 225, 226 Easterbrooks, M. A., 237, 248, 266 Eaton, W. O., 74, 93 Ebbesen, E. B., 207, 226 Eckerman, C. O., 248, 270 Edelstein, W., 210, 224

315

Edwards, C. P., 77, 78, 79, 81, 82, 83, 84, 93, 101 Egeland, B., 234, 235,236, 273, 274 Eibl-Eibesfeldt, I., 78, 82, 84, 93 Eichenbaum, H., 109, 148 Eisele, L. M., 94 Eisenberg, N., 195,201,202, 205, 206, 207, 208, 210, 211,212, 213, 214, 216, 217, 218, 219, 221,222, 223,224, 226, 228 Ekman, P., 190, 192, 223 Ellegren, H., 56, 99 Ellis, B. J., 71, 72, 93 Elmberg, J., 50, 97 Ember, C. R., 51, 63, 66, 67, 93, 98 Ember, M., 51, 66, 98 Emde, R., 237, 251,267, 273 Emlen, S. T., 45, 51, 52, 53, 93 Endler, J. A., 42, 44, 47, 56, 73, 93, 98, 99 England, B. G., 72, 74, 75, 93 Enns, L. R., 74, 93 Enquist, B. J., 44, 100 Eppler, M. A., 3, 4, 6, 9, 11, 21, 25, 29, 36, 37, 38 Epstein, J. L., 280, 308 Ergon, T., 50, 93 Ericsson, K. A., 33, 37 Erting, C., 249, 250, 270 Espinosa, M., 255, 256, 264, 267 Estes, W., 176, 182

Fabes, R. A., 195,201,206, 207, 208, 210, 211, 212, 213, 214, 216, 217, 218, 219, 221, 223, 224, 226, 228 Fagan, J. E, 104, 116, 148 Fagen, J. W., 104, 151 Fantz, R. L., 104, 148 Faraone, S. V., 214, 221,227 Farrant, K., 131,137, 148 Fein, D., 253,263 Feingold, A., 78, 93 Feiring, C., 240, 269 Feldman, J. F., 116, 151 Feldstein, S., 157, 183, 238, 240, 269 Fenner, M., 50, 93 Fenson, L., 119, 148 Fenwick, K., 157, 182, 185 Ferland, M. B., 157, 184

316

Author Index

Fernald, A., 166, 182, 199, 223 Ferrari, M., 293, 308 Field, T., 209, 224, 237, 247, 248, 249, 251, 259, 267 Fiese, B. H., 249, 271 Fisch, R. O., 248, 271 Fischer-Fay, A., 250, 273 Fisher, L. A., 287, 308 Fisher, P., 204, 206, 207, 227 Fitzgerald, R. W., 54, 94 Fitzhardinge, P., 248, 270 Fivush, R., 106, 114, 115, 128, 131,132, 137, 141,142, 147, 148, 149, 151 Flanagan, C., 257, 265 Flavell, E. R., 202, 223 Flavell, J. H., 202, 223 Fleeson, J., 237, 243,273 Flinn, M. V., 42, 51, 63, 65, 67, 69, 70, 71, 72, 73, 74, 75, 76, 78, 80, 86, 87, 88, 93, 94 Flom, R., 157, 171,172, 173, 181 Flood, M. F., 193,228 Fogel, A., 199, 200, 224, 225 Folkman, S., 195,204, 225 Follmer, A., 120, 123, 149 Folstad, I., 46, 48, 56, 59, 69, 74, 75, 93 Fonagy, P., 241,267 Forssberg, H., 5, 6, 37 Fowler, R. S., 250, 268 Fox, N. A., 209, 224, 228, 267 Fox, P. T., 123, 151 Fradin, S., 124, 148 Free, K., 259, 266 Freedman, D. G., 42, 93 Freeman, R. B., Jr., 109, 149 Freeseman, L. J., 169, 182 French, K. E., 74, 100 Frenkel, O. J., 249, 251,252, 255,274 Fretwell, S. D., 57, 62, 99 Freud, S., 105, 106, 148 Frick, J. E., 169, 182 Frick, P. J., 209, 226 Friedes, D., 154, 182 Friedman, J., 213,223 Friedman, S., 104, 148, 247, 267 Fritz, J., 200, 221 Frodi, A., 247, 248, 267 Fromhoff, F., 106, 131,141,148 Frost, D. O., 160, 182 Furman, W., 277, 308

Gadian, D. G., 109, 152 Gaffan, E. A., 270 Galbraith, S., 142, 152 Galpert, L., 253, 266 Gangestad, S. W., 69, 72, 73, 93, 94 Ganiban, J., 190, 222, 236, 251,252, 263,267 Garber, J., 203, 224 Gardner, W. L., 198, 221 Garrity-Rokous, E E., 259, 265 Gartner, D., 253, 267 Gartner, G., 248, 268 Gaulin, S. J. C., 54, 72, 73, 74, 94 Geary, D. C., 42, 45, 51, 54, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 80, 81, 85, 86, 87, 88, 89, 94 Geddie, L., 124, 148 Gelfand, D. M., 258, 259, 273 Gelman, R., 76, 94, 148 Genevro, J. L., 202, 203,220 Gentile, A. M., 33, 37 Gentzler, A. L., 193, 204, 222 George, C., 241,267 Georgieff, M. K., 145, 148 Gershoff, E. T., 210, 223 Gersten, M., 214, 221 Gervai, J., 77, 100 Gesell, A., 26, 28, 37, 38 Gianino, A., 199, 224 Giard, M. H., 160, 182 Gibson, E. J., 2, 4, 9, 10, 21, 24, 25, 26, 27, 36, 38, 39, 40, 155, 156, 157, 167, 177, 182 Gibson, J. J., 38, 154, 155, 177, 182, 183 Giedd, J., 209, 222 Gilbert, P., 44, 96 Gilger, J. W., 94 Gilliard, E. T., 54, 59, 94 Gleason, K. E., 242, 271 Gogate, L. J., 168, 183 Goldberg, N., 160, 181 Goldberg, S., 82, 94, 232, 235, 236, 246, 247, 248, 249, 250, 251,252, 254, 255,260, 263, 264, 266, 267, 268, 269, 273,274 Goldberg, W. A., 237, 266 Goldfield, E. C., 4, 38 Goldsmith, H. H., 122, 124, 130, 149, 199, 206, 208, 221,224, 243, 244, 268 Goodall, J., 64, 70, 83, 94, 95 Gorday, K. M., 25, 39

Author Index

Gordon, B. N., 120, 123, 139, 149 Gordon, E R., 24, 38 Gotowiec, A., 251,268 Gottesman, I. I., 84, 98 Gottlieb, G., 161, 185 Gottman, J., 203, 224, 227 Goubet, N., 25, 39 Granrud, C., 24, 40 Gray, J. A., 198, 224, 227 Gray, J. T., 106, 148 Green, F. L., 202, 223 Green, J. A., 200, 224 Greenberg, M. T., 246, 247, 248, 249, 250, 266, 268, 270 Greenhoot, A. E, 120, 123, 139, 149 Grolnick, W. S., 199, 200, 202, 204, 207, 208, 221,224 Gross, J. J., 196, 224 Gross, M. R., 59, 94 Grotpeter, J. K., 80, 86, 89, 92 Gruenwald, T. L., 67, 79, 80, 100 Grunebaum, H., 258, 270 Guendel, J., 106, 151 Guinness, F. E., 59, 92 Gunnar, M., 121,123, 124, 139, 140, 145, 149, 150, 196, 199, 224, 225, 240, 270 Gurung, R. A. R., 67, 79, 80, 100 Gustafson, G. E., 200, 224 Gustafsson, L., 48, 56, 98, 99 Gutfreund, M., 249, 250, 273 Guthrie, I. K., 195, 201,206, 207, 208, 210, 211,212, 213, 214, 216, 217, 218, 223, 224, 226, 228 Guttentag, M., 72, 94 Guy6tant, R., 50, 97

H Haas, E., 301,310 Haden, C. A., 131,132, 137, 141,142, 151 Haertel, 288, 309 Hakeem, A., 59, 64, 90 Halliday, T., 61, 98 Hallinan, M. T., 292, 299, 308 Hamalainen, M., 212, 213, 227 Hamilton, C. E., 236, 241,264, 268 Hamilton, W. D., 42, 45, 56, 94 Hammond, M. A., 247, 248, 263 Hamond, N. R., 114, 148

317

Hanburger, S., 209, 222 Hans, S. L., 257, 268 Hanson, B., 24, 40 Harlan, E., T., 201,207, 210, 212, 225 Harlow, H. F., 33, 35, 38 Harmon-Jones, E., 224 Harpending, H., 71, 72, 93 Harris, J. R., 77, 95, 304, 308 Harris, M. L., 199, 224 Harris, P. L., 202, 205, 224 Hart, D., 210, 224 Harter, N., 33, 37 Hartman, B., 24, 25, 39, 40 Hartup, W. W., 281,284, 292, 296, 303, 309 Harvald, B., 68, 95 Harvey, P. H., 57, 58, 59, 92, 95 Hasenstaub, A., 58, 69, 73, 74, 87, 90 Hassrick, R. B., 85, 95 Hauser-Cram, P., 247, 268 Hawkes, K., 64, 70, 85, 86, 90, 95 Hawkins, J. A., 297, 298, 301,307 Hayden, L. C., 258, 272 Hayden, T. J., 57, 97 Hayes, R., 168, 173, 180, 186 Hayne, H., 111, 114, 146 Haynes, O. M., 240, 269 Heath, A. C., 68, 69, 70, 96 Heckhausen, J., 195, 204, 224, 225 Hein, A., 24, 38, 161,183 Heinrich, J., 250, 269 Held, R., 24, 38, 161, 183 Hellewell, T. B., 184 Henderson, C., 11, 24, 37 Henggeler, S. W., 246, 249, 268 Hennig, K. H., 285, 310 Henson, L. G., 257, 268 Herbsman, C. R., 206, 229 Herman-Giddens, M. E., 72, 95 Hermelin, B., 253,268 Hernandez-Reif, M., 157, 168, 183 Hershey, K. L., 121, 151,204, 206, 207, 212, 227 Herskind, A. M., 68, 95 Hertsgaard, L., 112, 114, 115, 117, 118, 119, 147, 199, 224 Hertzig, M. E., 253, 273 Hesketh, G. L., 85, 96 Hesse, E., 238, 270 Hetherington, W. M., 305, 308 Hewlett, B. S., 84, 95

318

Author Index

Hiatt, S., 11, 24, 37 Hill, C. E., 44, 96 Hill, D., 253,266 Hill, K., 42, 63, 64, 65, 66, 67, 68, 69, 70, 76, 84, 86, 88, 95 Hinde, R. A., 268 Hines, M., 79, 82, 83, 90, 92, 95 Hirata, M., 110, 152 Hirsch, B. J., 297, 309 Hirschi, T., 300, 309 Hirshfeld, D. R., 214, 221,227 Hoang, A., 44, 96 Hoard, M. K., 72, 94 Hobbs, N., 268 Hobson, R. P., 252, 268 Hoekstra, H. E., 44, 96 Hoekstra, J. M., 44, 96 Hofer, M. A., 50, 95 Hoffner, C., 203,225 Hofmann, V., 210, 224 Hofsten, C., 2, 25, 38, 40 H6glund, J., 56, 57, 61, 95 Holm, N. V., 68, 95 Holmgren, R., 195,206, 207, 216, 218, 223, 224 Holt-Lunstad, J., 295, 296, 310 Hommerding, K. D., 236, 274 Honeycutt, H., 173, 174, 177, 178, 184 Honeycutt, W. S., 161, 186 Hopkins, B., 158, 181 Hornik, R., 199, 225 Horowitz, D. L., 65, 95 Horsborough, K. M., 249, 250, 271 Houle, S., 109, 147, 149 Houston, A. I., 50, 97 Howard, J., 248, 255,256, 257, 264, 267, 271, 273 Howe, M. L., 120, 142, 149 Howes, C., 277, 309 Hubbard, J. A., 207, 225 Hudson, J. A., 106, 114, 115, 131,137, 149, 151 Huffman, K. J., 76, 94 Hurtado, A. M., 63, 64, 65, 66, 67, 68, 69, 70, 76, 84, 86, 88, 95 Husarek, S. J., 225 Hutt, C., 116, 149 Hyche, J. K., 247, 268 Hyde, J. S., 68, 98 Hymel, S., 300, 309

I

Ide, J. K., 288, 309 Ignatoff, E., 251,267 Ikejiri, Y., 110, 152 Inhelder, B., 154, 185 Innocenti, G. M., 160, 183 Insel, T. R., 64, 99 keys, H. T., 268 Irons, W., 86, 95 Isabella, R. A., 237, 238, 240, 258, 259, 268, 273 Isenberg, D. J., 281,309 Izard, C. E., 191,192, 225,240, 269

J

Jacklin, C. N., 77, 97 Jacobs, B. S., 247, 267 Jacoby, L. L., 110, 148, 149 Jacques, T. Y., 212, 225 Jaffe, J., 157, 183, 238, 240, 269 James, W., 155, 183 Janis, I. L., 282, 309 Janowsky, J. S., 109, 110, 149 Jarvis, P. A., 248, 269 Jasnow, M. D., 157, 183,238, 240, 269 Jay, M. F., 160, 183 Jennings, K. D., 83, 95 Jetter, W., 109, 149 Jiao, Z., 297, 298, 301,307 Joffe, T. H., 59, 69, 71, 86, 95 John, O. P., 210, 211,227 Johnson, D. W., 79, 89 Johnson, J. A., 162, 185 Johnson, M. L., 6, 38 Jokela, J., 76, 95 Jones, C., 109, 149 Jones, O., 247, 269 Jones, S., 195, 213, 214, 217, 223 Joseph, R. M., 253,269 Juhnke, C., 212, 223

K

K ~ , P., 76, 95 Kagan, J., 197, 208, 214, 225,227, 243, 244, 269

319

Author Index

Kail, R. V., 120, 149 Kandel, D. B., 286, 287, 288, 309 Kandel, E. R., 109, 149 Kanner, L., 252, 253, 269 Kaplan, H., 42, 63, 64, 65, 66, 67, 68, 69, 70, 76, 84, 86, 88, 95 Kaplan, N., 241,242, 267, 270 Kaplowitz, H., 74, 97 Kapur, S., 109, 147, 149 Karbon, M., 212, 213, 219, 223 Karter, A. J., 46, 48, 56, 59, 69, 74, 75, 93 Kasari, C., 251,253, 269, 271,272, 274 Katz, E., 251,267 Kaufman, E R., 79, 95 Kaye, K., 200, 225 Keating, M. J., 160, 161, 187 Keefe, K., 276, 278, 279, 280, 288, 289, 296, 297, 298, 299, 301,307, 309 Keefe, R. C., 71, 95 Keeley, L. H., 65, 66, 74, 95 Keenan, K., 292, 309 Keener, A. D., 198, 209, 229 Keener, M. A., 247, 263 Keitner, G., 258, 272 Keller, M., 210, 224 Kellerman, E J., 159, 183 Kelly, S., 69, 74, 95 Kemp, K. J., 62, 91 Kenrick, D. T., 42, 63, 71, 95, 96 Kermoian, R., 24, 27, 37, 191,194, 195, 201, 221,222 Kerns, K. A., 193, 204, 222 Kestenbaum, R., 240, 270 Killen, M., 284, 308 Kimura, D., 74, 84, 96, 100 Kinchla, R., 176, 183 Kindlon, D., 197, 210, 212, 225, 226 King, A. J., 159, 161,183 Kingsnorth, S., 26, 38 Kingsolver, J. G., 44, 96 Kirk, K. M., 68, 69, 70, 96 Kirkland, J., 237, 271 Kirkwood, T. B. L., 66, 100 Klein, L. C., 67, 79, 80, 100 Kleinknecht, E. E., 125, 127, 129, 130, 134, 137, 140, 144, 149 Kliewer, W., 201,225 Klimes-Dougan, B., 259, 266 Knight, G. E, 79, 96 Knowlton, B., 108, 109, 152

Knudsen, E. I., 160, 161,183 Koch, D., 272 Koch, G., 72, 95 Kochanska, G., 201,207, 208, 210, 212, 225 Kochenderfer, B. J., 297, 298, 309 Koella, J. C., 50, 62, 69, 70, 72, 85, 89, 100 Koenig, A. L., 212, 225 Kogan, K. L., 246, 269 Kogan, N., 193, 204, 212, 222 Kokko, H., 57, 96 Kolakowski, D., 74, 96 Kopp, C. B., 194, 198, 199, 200, 201,202, 225 Kornblatt, E. S., 250, 269 Korpelainen, H., 69, 72, 73, 96 Koslowski, B., 238, 264 Kozuch, E, 209, 222 Kraemer, E J., 161, 173, 184 Krampe, R. T., 33, 37 Krauss, M. W., 247, 268 Krebs, J. R., 52, 96 Kremen, A. M., 221 Krettenauer, T., 285, 310 Kriger, A., 82, 94 Kroonenberg, E M., 249, 251,252, 255, 274 Kroupina, M. G., 108, 119, 144, 145, 147, 150 Krowitz, A., 24, 37 Krueger, R. E, 207, 212, 225 Krupka, A., 247, 270 Kuczynski, L., 258, 271 Kuhl, E K., 157, 183 Kuhn, D., 183 Kumar, R., 58, 69, 73, 74, 87, 90 Kurowski, C. O., 202, 204, 224 Kuurman, W. W., 70, 92 Kwon, J., 253, 272

L Ladd, G. W., 297, 298, 309 LaFrenier, E, 77, 91 Lamb, M., 199, 221,247, 251,267, 273 Lambin, X., 50, 93 Lampert, W., 50, 90 Lampl, M., 6, 38 Lampman-Petraitis, C., 220, 225 Lancaster, C. S., 63, 65, 70, 96 Lancaster, J., 63, 64, 65, 66, 67, 68, 69, 70, 76, 84, 86, 88, 95, 96 Landry, S. H., 252, 253,269

320

Author Index

Lang, S., 240, 270 Langer, A., 24, 37 Larsen, R. J., 69, 71, 91, 99 Larsen, U., 69, 70, 97 Larson, M. C., 199, 224 Larson, R., 220, 225 Laychak, A. E., 282, 284, 285,307 Lazarus, R. S., 195,204, 225 Leadbeater, B. J., 80, 96 Leavitt, L., 247, 267 Le Boeuf, B. J., 57, 58, 92, 96 Lecanuet, J.-P., 175, 182 Lederberg, A. R., 249, 250, 269 Lee, D. N., 4, 25, 38, 158, 186 Lee, P. C., 81, 96 Leen, T. K., 3, 4, 6, 38 Leerkes, E., 259, 266 Lefford, A., 154, 181 Leigh, S. R., 58, 73, 74, 87, 96 Lengua, L. J., 212, 225 Lennon, E., 157, 187 Lennon, M. C., 250, 274 Leo, A. J., 9, 38 Lepage, M., 150 Leslie, A. M., 76, 96 Lever, J., 78, 88, 96 Leveroni, C., 82, 96 Levison, 250, 268 Levy, A. K., 235, 273 Lewis, B. P., 67, 79, 80, 100 Lewis, M., 240, 251,265, 269 Lewkowicz, D. J., 155, 157, 161,162, 173, 179, 183, 184, 185 Liang, J., 280, 292, 293, 310 Lickliter, R., 155, 157, 158, 159, 161,164, 166, 170, 171,172, 173, 174, 177, 178, 179, 180, 181,182, 183, 184, 185, 186 Liew, J., 212, 223 Ligon, J. D., 56, 101 Lishman, J. R., 4, 38 Little, C., 193, 204, 212, 222, 259, 265 Lo, T., 25, 38 Locke, J., 154, 184 Lockman, J. J., 25, 27, 38 Loeber, R., 292, 309 Logan, G., 176, 184, 201,219, 225, 229 Lojkasek, M., 248, 251,268, 269 Losoya, S., 195, 207, 208, 210, 213, 214, 216, 217, 218, 223, 228 Loveland, K. A., 252, 253, 269

Low, B. S., 67, 71, 73, 85, 89, 93, 96 Loy, J. W., 85, 96 Lu, Y., 6, 30, 31, 37 Luce, C. L., 42, 63, 96 Lumley, J. M., 249, 274 Lummaa, V., 97 Lundquist, L., 193,228 Luthar, S. S., 257, 269 Lynch, J. S., 142, 152 Lyons-Ruth, K., 235, 236, 239, 258, 269, 270

M

Mac Arthur, R. H., 49, 57, 59, 97 Maccoby, E. E., 77, 78, 79, 86, 88, 97, 303, 304, 305, 306, 308, 309 MacDonald, J., 162, 184 MacDonald, K., 71, 85, 97 MacGregor, D., 251,269, 274 MacLean, D. J., 172, 182 Maclusky, I., 250, 273 Maegeais, R., 175, 182 Mahler, M. S., 237, 270 Main, M., 235, 238, 241,242, 264, 267, 270 Maiti, A. K., 209, 227 Malatesta, C. Z., 191,192, 193, 226 Malina, R. M., 74, 96 Malkin, C., 251,273 Mandler, J. M., 97, 104, 108, 111,114, 147, 150 Mangelsdorf, S., 193, 199, 200, 204, 208, 222, 226, 240, 270 Manning, J. T., 62, 72, 97, 99 Marcovitch, S., 251,269, 274 Marfo, K., 247, 270 Matin, L., 3, 6, 30, 31, 36, 37 Markon, K., 80, 86, 89, 92 Markowitsch, H. J., 109, 149, 150 Marks, L. E., 154, 184 Marshall, R. E., 209, 227 Martin, J. M., 303,304, 306, 309 Martin, N. G., 68, 69, 70, 96 Martin, R. P., 123, 150 Martins, C., 270 Martorell, R., 74, 97 Marvin, R. S., 248, 250, 251, 261,268, 270 Marzolf, D., 199, 200, 226 Maslin, C., 240, 241,264 Maslin-Cole, C., 254, 271,272 Massaro, D. W., 161,184

Author Index

Masse, L. C., 290, 310 Massop, S. A., 25, 38 Maszk, P., 195, 206, 207, 213, 216, 218, 219, 223, 224 Mathur, P., 31, 36 Matthews, M. H., 83, 86, 97 Mayr, E., 43, 76, 97 McCall, R. B., 268 McCartney, K. A., 282, 285, 286, 307 McCaskill, C. L., 26, 38 McClear, P. M., 74, 90 McClearn, G. E., 44, 98 McCleary, K., 283,307 McCollum, G., 3, 4, 6, 38, 39 McCord, J., 284, 300, 301,306, 308 McDevitt, S. C., 123, 147 McDonough, L., 108, 114, 150 McDougall, P., 300, 309 McElligott, A. G., 57, 97 McFayden-Ketchum, S., 71, 72, 93 McGehee, L. J., 248, 270 McGraw, M., 26, 28, 29, 39 McGue, M., 68, 95 McGuffin, P., 44, 98 McGurk, H., 162, 184 McHenry, H. M., 65, 97 Mclntosh, A. R., 109, 147 McKee, R. D., 108, 150 McKenzie, B. E., 24, 25, 39 McKinzie, D. L., 173, 186 McLaren, I. A., 50, 97 McLaughlin, T., 59, 64, 90 McLeod, S., 236, 270 McMahon, T. J., 257, 269 McMenamy, J. M., 202, 204, 224 McNamara, J. M., 50, 97 McPherson, M., 280, 286, 309 Meadow, K. P., 249, 250, 270, 272 Meerum, 202, 205, 224 Meisels, S. J., 248, 249, 271 Mellon, R. C., 161,173, 184 Meltzoff, A. N., 107, 108, 110, 114, 118, 142, 150, 157, 183, 184, 253, 266 Mendelson, M. J., 157, 184, 289, 307 Mendoza-Denton, R., 212, 221 Menken, J., 69, 70, 97 Meredith, M. A., 159, 160, 161,176, 178, 184, 186, 187 Meril~i, J., 56, 99 Merrick, S., 274

321

Messinger, D. S., 258, 259, 273 Metcalfe, J. S., 25, 39 Mettetal, G., 203, 224 Meyer-Bahlburg, H. E L., 81, 99 Meyers, T., 257, 258, 265 Mezzacappa, E., 197, 210, 212, 225, 226 Miaud, C., 50, 97 Michel, G. F., 165, 184 Michel, M. K., 190, 197, 222 Miller, I., 258, 272 Miller, J. O., 176, 185 Miller, K. E., 283,307 Minch, E., 66, 99 Minde, K., 248, 268, 270 Mirsky, A. E, 198, 226 Mischel, H. N., 202, 226 Mischel, W., 202, 207, 208, 212, 221,226, 228 Mishkin, M., 109, 146, 150, 152 Mistlin, A. J., 160, 185 Mobley, C. E., 249, 250, 269 Moffitt, T. E., 207, 208, 209, 210, 211, 212, 220, 225, 226, 227, 229 Mr A. P., 48, 56, 71, 97, 99 Moore, A. M., 282, 285, 286, 307 Moore, C. L., 165, 184, 185 Moran, G., 239, 242, 243, 247, 270, 271 Morgan, E. E, Jr., 81,100 Morgan, R., 158, 185 Morison, V., 24, 39 Morris, A. S., 205, 216, 217, 223 Morris, E O., 55, 97 Morris, P., 250, 268 Morris, R., 253, 263 Morrongiello, B. A., 157, 182, 185 Morrow, J., 123, 139, 152 Moscovitch, M., 106, 109, 110, 150, 151 Moses, L. J., 199, 226 Mosher, M., 67, 69, 79, 89, 92 Mousseau, T. A., 43, 44, 97 Mullen, M. K., 150 Muller, J., 154, 185 Mumme, D. L., 191,193,194,195,199,222,228 Mundy, P., 235,253, 254, 265,269, 271,272, 274 Munroe, R. H., 83, 97 Munroe, R. L., 83, 97 Murdock, G. P., 70, 82, 84, 97 Murphy, B., 195, 201,202, 206, 207, 208, 210, 212, 213, 214, 216, 217, 218, 223, 224, 226, 228

322

Author Index

Murphy, M., 213, 219, 223 Murray, A., 31, 36 Murray, E. A., 109, 150 Murray, K., 201,207, 208, 210, 212, 225 Murray, L., 258, 271 Musen, G., 108, 109, 152 Myers, B. J., 248, 269

N

Nakagawa, Y., 110, 152 Nashner, L. M., 4, 5, 6, 37, 39 Nawrocki, T., 267 Needles, D. J., 208, 209, 229 Neff, C., 247, 267 Nelson, C. A., 105, 110, 111,122, 124, 126, 137, 139, 140, 145, 147, 148, 149, 150 Nelson, D. A., 80, 86, 89, 92 Nelson, K., 106, 131,141,151,152 Neufeld, S. J., 198, 200, 201,225 Newcomb, A. F., 78, 97 Newman, D. L., 210, 226 Newman, H. W., 110, 146 Newman, J. P., 209, 227 Niccols, A., 258, 263 Nicholls, A., 237, 271 Nickerson, R., 176, 185 Nicolson, N. A., 81, 97 Nida, R. E., 120, 123, 149 Nienhuys, T. G., 249, 250, 271 Nigg, J. T., 197, 226 Niles, D., 26, 40, 57, 62, 99 Nishikawa, T., 110, 152 Nordling, D., 48, 98 Norris, D., 233,264 Nowicki, S., 98 Nunney, L., 48, 99 Nutley, T., 157, 185

O O'Boyle, C. G., 200, 227 O'Brien, B. S., 209, 226 O'Brien, K. M., 80, 86, 89, 92 O'Connell, J. E, 64, 70, 85, 86, 90, 95 O'Connor, J. J., 257, 271 O'Connor, M. J., 257, 271 O'Connor, N., 253, 268

Oliver, M. B., 68, 98 Olson, S. L., 208, 212, 226 Olthof, T., 202, 205, 224, 228 Oosterlaan, J., 212, 226 Oring, L. W., 52, 53, 93 Ornstein, P. A., 120, 123, 139, 149 Ortegon, J., 253, 269 Osterling, J., 253, 266, 271 Owens, I. P. F., 68, 69, 70, 96 Ozonoff, S., 254, 271,272

P

Paglia, A., 258, 263 Palmer, A. R., 161, 183 Palmer, C. E., 6, 39 Palmer, C. E, 26, 39 Papoport, J. L., 209, 222 Pardue, M.-L., 72, 73, 74, 101 Park, K., 282, 283, 284, 285, 307 Parke, R. D., 303, 309 Parker, G. A., 52, 53, 98 Parker, J. G., 203, 227 Parker, S. W., 145, 150 Parkerson, J., 288, 309 Parmelee, A. H., 233,264, 272 Parsons, E., 239, 270 Partridge, L., 56, 98 Pasick, E L., 248, 249, 271 Pasternak, B., 51, 66, 98 Pattee, L., 78, 97 Patterson, C. M., 209, 227 Patterson, G. R., 305, 308, 309 Patton, G., 80, 91 Peake, E K., 208, 212, 221,226, 228 Pearson, D. A., 253, 269 Pearson, J. L., 259, 271 Pearson, E L., 70, 92 Pederson, D., 239, 242, 243, 247, 270, 271 Pellegrini, A. D., 42, 63, 76, 77, 78, 86, 88, 90, 98 Peronnet, E, 160, 182 Perrett, D. I., 160, 185 Perrin, J. M., 268 Perrotta, M., 248, 268 Perry, T. B., 295, 307 Person, E., 214, 225 P6russe, D., 67, 74, 98 Peters, S., 98

323

Author Index

Petersen, S. E., 123, 151 Pethick, S. J., 119, 148 Pethkongathan, S., 29, 36 Petrie, M., 61, 62, 98 Pettit, G. S., 71, 72, 93 Pettit, E, 247, 270 Phillipsen, L., 233, 264 Piaget, J., 27, 39, 104, 105, 106, 107, 139, 142, 151,154, 185, 283, 285, 295, 309 Pianta, R. C., 250, 251, 261,270 Pick, A. D., 2, 38, 156, 182 Pickens, J., 154, 155, 157, 181,185, 267 Picketing, A. D., 198, 227 Pidada, S., 212, 223 Pien, D. E, 201,207, 227 Pilgrim, C., 292, 293, 310 Pillemer, D. B., 105, 151 Pine, E, 237, 270 Pinker, S., 64, 98 Pipp-Siegel, S., 235, 271 Pitcher, E. G., 79, 81, 86, 98 Plavcan, J. M., 57, 65, 98 Plomin, R., 44, 98, 268 Plunkett, J. W., 248, 249, 271 Podos, J., 98 Poehlmann, J., 249, 271 Pogge-Hesse, E, 247, 265 Polazzi, L., 212, 223 Pollitt, E., 74, 97 Ponesse, J. S., 201,229 Popper, S., 257, 265 Porges, S. W., 209, 227, 228 Porter, E L., 209, 227 Poser, U., 109, 149 Posner, M. I., 122, 123, 127, 151,152, 198, 201,220, 227 Poulin, E, 284, 286, 300, 301,306, 308, 310 Poulin, R., 195, 213,214, 217, 223 Price, G. R., 42, 98 Pryce, C. R., 81, 98 Pulkkinen, L., 212, 213, 227 Pusey, A., 64, 70, 95

O Quinlan, D. M., 80, 96 Quinlan, R. J., 72, 74, 75, 93 Quinn, P. C., 168, 173, 180, 186

R

Raab, D., 176, 185 Radell, E, 161, 185 Rademacher, J., 125, 127, 129, 130, 134, 137, 140, 149 Rader, N., 24, 26, 27, 39 Radke-Yarrow, M., 258, 259, 266, 271 Ragozin, A. S., 246, 247, 248, 266 Raichle, M. E., 123, 151 Ramsay, D., 11, 24, 37 Rasmussen, S., 258, 272 Ratner, H. H., 106, 151 Rawlins, W. K., 310 Reed, E. S., 2, 25, 39 Reed, M. A., 122, 148, 201,207, 227 Reese, E., 131,132, 137, 141,142, 148, 151 Reid, J. B., 305, 309 Reinisch, J. M., 116, 151 Reiser, M., 195,207, 208, 210, 211,212, 213, 214, 216, 217, 218, 222, 223 Reiter, J., 58, 96 Renznick, J. S., 119, 148 Repacholi, B., 236, 270 Resnick, S., 84, 98, 245, 272 Reynolds, G. D., 178, 185 Reynolds, J. D., 53, 54, 57, 98 Reznick, D., 42, 44, 47, 48, 73, 98, 99 Riccio, G., 4, 25, 38, 39 Richards, J. E., 24, 26, 27, 39, 209, 227 Richardson, J. T. E., 110, 146 Richer, E, 150 Richner, H., 99 Ridgeway, D., 200, 221,240, 241,264 Rilling, J. K., 64, 99 Rinaldi, J., 253, 266 Riordan, K., 245, 272 Risenhoover, N., 199, 225 Rivera, J., 74, 97 Rivkin, I., 202, 204, 224 Roberts, W., 227 Robertson, S. S., 175, 185 Robins, R. W., 210, 211,227 Robinson, J., 237, 238, 264 Robinson, N. M., 246, 247, 248, 266 Rochat, E, 25, 39, 157, 158, 185 Rodd, E H., 48, 99 Rode, S. S., 248, 271 Rodning, C., 233, 248, 255, 256, 264, 271 Rodriguez, M., 212, 221,226

324

Author Index

Roff, D. A., 41, 43, 44, 46, 47, 48, 49, 50, 51, 57, 59, 73, 87, 97, 99 Rogers, S. J., 254, 271,272 Rogosch, F. A., 258, 265 Rohwer, S., 57, 62, 99 Roloff, D. W., 248, 249, 271 Rosblad, B., 25, 40 Rose, A. J., 79, 80, 89, 92, 99 Rose, J. L., 25, 27, 37 Rose, S. A., 116, 151,157, 185 Rosenbaum, J. E, 214, 221,227 Rosenberg, B. G., 81, 100 Rosenberg, D., 25, 38 Rosenblum, L. A., 116, 151 Rosenblum, L. D., 162, 185 Rosicky, J. G., 199, 226 Rosin, A., 58, 69, 73, 74, 87, 90 Ross, S., 265 Rothbart, M. K., 121, 122, 123, 127, 130, 148, 149, 151,152, 195, 197, 198, 200, 201, 204, 205,206, 207, 208, 212, 214, 218, 220, 222, 224, 227, 243,244, 268, 272 Rothbaum, E, 243, 255,272 Rovee, C., 104, 151 Rovine, M., 238, 240, 264 Rozga, A., 253, 254, 272 Rubin, D. B., 116, 151 Rubin, D. C., 105, 151 Rubin, K., 80, 91 Ruble, D. N., 202, 203, 220 Rudder, B., 58, 92 Rudolph, K. D., 203,227 Rumelhart, D., 176, 185 Ruskin, E., 253, 272 Rutter, M., 253, 272 Ryan, N. M., 201,227 Ryan, R. M., 228

S Saarni, C., 193, 194, 199, 202, 205, 207, 228 Saino, N., 48, 56, 99 Salapatek, P., 21, 28, 39 Salovey, P., 198, 221 Salthouse, T. A., 120, 149 Saltzman, H., 206, 222 Sameroff, A. J., 232, 245,258, 272 Sandberg, D. E., 81, 99

Sander, L. W., 199, 228 Sanders, C., 61, 98 Sandier, I. N., 195, 204, 212, 225, 228 Santos, A. J., 77, 100 Sathian, K., 162, 185 Satterwhite, T., 11, 29, 37 Saul, J. P., 197, 210, 226 Savin-Williams, R. C., 78, 79, 99, 280, 296, 307, 310 Sawaguchi, T., 59, 88, 99 Scarr, S., 21, 28, 39, 304, 310 Schachar, R. J., 201, 219, 225,229 Schacter, D. L., 106, 110, 151 Schier, C., 162, 185 Schiller, M., 240, 243, 244, 245,272 Schilling, E. M., 208, 212, 226 Schlesinger, H. S., 249, 272 Schmidt, R. A., 33, 34, 39 Schmitt, D. P., 67, 91 Schmuckler, M. A., 4, 24, 25, 26, 38, 39, 162, 185 Schmuckler, M. J., 158, 185 Schmuckler, M. S., 26, 38 Schneider, W., 116, 151,176, 186 Schneider-Rosen, K., 243, 255,272 Schonert-Reichl, K., 300, 309 Schuengel, C., 239, 259, 272, 274 Schulsinger, M. F., 116, 151 Schultz, L. H., 79, 81, 86, 98 Schultz, N. M., 253, 267 Schulz, R., 195,225 Schunk, D. H., 284, 310 Schwade, J. A., 108, 119, 144, 147 Scott, B., 235, 236, 251,252, 254, 263 Scutt, D., 72, 99 Secord, P., 72, 94 Seielstad, M. T., 66, 99 Seifer, P., 251,274 Seifer, R., 232, 240, 243,244, 245, 258, 272 Sekuler, R., 177, 186 Selander, R. K., 62, 99 Senders, S. J., 25, 39 Serafica, F. C., 251,265 Sergeant, J. A., 212, 226 Serres, L., 110, 151 Seyfarth, R. M., 62, 100 Shackelford, T. K., 67, 69, 91, 99 Shams, L., 162, 186 Shapira, T., 272 Shapiro, J., 199, 200, 226, 272

Author Index

Shapiro, T., 253, 273 Shaw, D. S., 236, 274 Shaw, E H., 48, 99 Shaw, M., 176, 186 Shaw, R. G., 48, 99 Sheffield, E. G., 114, 149, 151 Sheldon, B. C., 56, 57, 66, 95, 99, 100 Sheldon, M. S., 72, 94 Sheng, Y. Y., 25, 39 Shepard, S., 195, 201,202, 207, 208, 210, 213, 214, 217, 218, 223, 226, 228 Shephard, B., 191,192, 193, 226 Sherif, C., 283, 310 Sherif, M., 283, 310 Sherman, 240, 241,264 Sherman, M., 272 Sherman, T., 253, 271,272 Sheryy, D., 247, 267 Shields, A., 204, 228 Shiffrin, R., 176, 186 Shilansky, M., 250, 274 Shimamura, A. P., 109, 110, 149 Shimojo, S., 162, 185 Shine, R., 49, 99 Shinozaki, K., 110, 152 Shoda, Y., 208, 212, 226, 228 Shonkoff, J. P., 247, 268 Shuman, H. H., 248, 267 Shumway-Cooke, A., 25, 39 Shyu, S. J., 280, 292, 293, 310 Siegel, C. H., 235, 271 Siegler, R. S., 83, 91, 183 Sigelman, J., 224 Sigman, M., 233, 235, 251,253, 254, 257, 265, 269, 271,272, 274 Silk, J. B., 62, 99 Silva, E., 236, 270 Silva, P. A., 210, 226 Silverman, I., 83, 99 Simmons, L. W., 52, 53, 98 Simmons, R. J., 250, 251,268, 273 Sinervo, B., 48, 50, 57, 72, 99 Singh, D., 71,100 Sitko, C., 239, 271 Skinner, E. A., 195, 228 Skolowski, M. B., 48, 99 Skouteris, H., 24, 25, 39 Slabach, E. H., 123, 139, 152 Slater, A., 24, 39, 168, 173, 180, 186 Sleigh, M. J., 178, 186

325

Sloman, J., 246, 247, 248, 264 Smith, K. D., 208, 222 Smith, L. B., 24, 27, 34, 40, 179, 186 Smith, M., 213, 219, 223 Smith, M. C., 198, 229 Smith, P. K., 58, 77, 78, 88, 98, 100 Smith-Lovin, L., 280, 286, 309 Smuts, B. B., 58, 100 Snidman, N., 197, 214, 225 Snow, M. E., 253, 273 Snyder, E., 79, 82, 90 Soler, M., 71, 97 Solomon, C. R., 246, 274 Solomon, J., 235, 270 Sorce, J. F., 251,273 SCrensen, T. I. A., 68, 95 Sparks, D. L., 160, 183 Spear, N. E., 161,173, 184, 186 Spelke, E. S., 157, 186 Spence, C., 159, 160, 182 Spence, M. J., 175, 182 Spencer, A., 253, 266 Spencer, P. E., 249, 250, 273 Spiker, D., 237, 239, 266 Spinrad, T. L., 08, 205, 207, 208, 213, 214, 216, 217, 218, 223, 228 Spracklen, K. M., 308 Squire, L. R., 108, 109, 110, 149, 150, 151,152 Sroufe, L. A., 192, 228, 234, 235, 236, 237, 243, 244, 248, 251,265, 271,273, 274 Stagner, B., 293, 310 Stahl, J., 235, 270 Stahlecker, J. E., 273 Stamps, J. A., 57, 100 Steams, S. C., 46, 48, 49, 50, 51, 57, 58, 62, 69, 70, 72, 85, 89, 100 Steele, H., 241,267 Steele, M., 241,267 Stein, B. E., 159, 160, 161,162, 176, 178, 182, 184, 186, 187 Steinberg, L., 71, 90, 278, 305, 308, 310 Stenberg, C., 199, 221 Stenseth, N. C., 50, 93 Stern, D., 157, 186, 237, 238, 273 Sternberg, R. J., 293,308 Stice, E., 213, 222 Stiefel, G. S., 248, 249, 271 Stifter, C. A., 200, 209, 228 Stipek, D. J., 253,269 Stoffregen, T., 4, 25, 38, 39, 154, 155, 186

326

Author Index

Stouthamer-Loeber, M., 207, 208, 209, 210, 211, 212, 225, 227, 229, 292, 309 Strayer, E E, 77, 100 Streri, A., 157, 186 Striano, D., 157, 185 Stringer, S., 251,267 Sugita, Y., 110, 152 Suh, K., 206, 207, 224 Sullivan, H. S., 294, 296, 302, 310 Sultan, S. E., 50, 62, 100 Suttle, G. D., 310 Sutton-Smith, B., 81,100 Sveistrup, H., 25, 40 Svejda, M., 11, 24, 37 Svensson, E., 48, 50, 57, 66, 72, 99, 100 Swanson, K., 255,256, 257, 267, 273 Symons, D., 68, 100 Sz6kely, T., 57, 98

T Tager-Flusberg, H., 253, 269 Takeda, M., 110, 152 Takimoto, H., 110, 152 Tam, F., 235,236, 251,252, 254, 263 Tanabe, H., 110, 152 Taniguchi, M., 110, 152 Tanner, J. M., 74, 100 Tannock, R., 201,219, 225,229, 247, 251,273 Taormina, J., 25, 38 Tarulla, L. B., 259, 266 Taylor, D. G., 238, 240, 264 Taylor, S. E., 67, 79, 80, 100, 196, 221 Tein, J., 195, 204, 228 Tellegen, A., 195, 228 Terwogt, M. M., 202, 205,224, 228 Tesch-Romer, C., 33, 37 Tesman, J. R., 191, 192, 193, 226 Tessier, A., 48, 99 Tessler, M., 131, 141, 152 Teti, D., 258, 259, 273 Teti, L. O., 190, 197, 222 te Velde, E. R., 70, 92 Thai, D. J., 114, 119, 147, 148 Thelen, E., 24, 25, 26, 27, 34, 39, 40, 179, 186, 199, 224 Thomas, A., 121,152, 244, 265, 268 Thomas, J. R., 74, 100 Thomas, L., 80, 91

Thompson, H., 26, 38 Thompson, E, 110, 146 Thompson, R., 247, 248, 267 Thompson, R. A., 190, 191,193, 196, 199, 203, 207, 228, 251,273 Thomsen, A. H., 206, 222 Thornhill, R., 56, 69, 71, 72, 73, 93, 94, 97, 101 Thorpe, J., 251,267 Thrasher, F. M., 282, 310 Tidball, G., 199, 226 Tiger, L., 66, 74, 100 Titzer, R., 11, 40 Tjebkes, T. L., 225 Tomarken, A. J., 198, 209, 229 Tomich, P. L., 193, 204, 222 Toth, S. L., 258, 265 Trainor, R., 209, 222 Travis, L. L., 114, 128, 147 Treboux, D., 274 Tremblay, R. E., 290, 310 Trevarthan, C., 157, 186 Trivers, R. L., 51, 52, 54, 58, 65, 66, 70, 89, 100 Tronick, E., 157, 186, 199, 224, 238, 240, 266 Trussell, J., 69, 70, 97 Tulving, E., 109, 147, 149 Tunali-Kotoski, B., 253, 269 Turkewitz, G., 161, 184 Turner, M. T., 72, 74, 75, 93 Turner, P. J., 77, 100 Tyler, N., 246, 269 Tyler, R., 255, 256, 264, 267

Uchino, B. N., 295,296, 310 Udry, J. R., 289, 310 Ulrich, B., 26, 40 Underwood, M. K., 206, 207, 229 Ungerer, J., 253,271,272 Uno, D., 295, 296, 310 Updegraff, J. A., 67, 79, 80, 100 Upton, D., 110, 146 Urberg, K. A., 280, 292, 293, 310

Vaituzis, C. K., 209, 222 Van Abbema, D. L., 120, 139, 144, 146, 147, 152

Author Index

van Aken, M. A. G., 210, 221 van Arendonk, J. A. M., 70, 92 Vandegeest, K. A., 212, 225 Van de Meer, A. L., 26, 40, 158, 186 van den Boom, D., 243, 245, 273 Van der Weel, E L., 158, 186 van Engeland, H., 254, 274 van IJzendoorn, M. H., 237, 239, 241,242, 249, 251,252, 254, 255, 259, 266, 272, 273,274 Van Kammen, W. B., 292, 309 van Noord, P. A. H., 70, 92 Van Paesschen, W., 109, 152 van Schaik, C. P., 57, 65, 98 Vargha-Khadem, E, 109, 152 Vaughn, B. E., 235, 236, 243, 244, 251,252, 254, 263, 274 Vaupel, J. W., 68, 95 Vauss, Y., 209, 222 Vecera, S. P., 123, 152 Veldhuis, J. D., 6, 38 Vellenga, R. E., 60, 100 Vereijken, B., 6, 27, 28, 36 Vignieri, S. N., 44, 96 Vincent, A. C. J., 52, 92 Vinokur, A., 285, 310 Vitaro, E, 290, 310 Vondra, J. I., 235, 236, 260, 263, 274 von Eye, A., 238, 240, 268

W

Wachs, T. D., 123, 139, 152 Wade, L., 161,186 Wadsworth, M. E., 206, 222 Walberg, H.J., 288, 309 Walchli, S. B., 283,284, 290, 308 Walden, T. A., 198, 229 Walk, R. D., 10, 24, 27, 38, 40 Walker, A. S., 157, 182, 186 Walker, C., 45, 91 Walker, L. J., 285, 310 Walker-Andrews, A. S., 155, 156, 157, 158, 159, 172, 173, 186, 187 Wall, S., 233, 234, 236, 237, 239, 240, 241, 244, 260, 262, 263 Wallace, A., 42, 43, 92 Wallace, I. F., 116, 151

327

Wallace, M., 161, 186 Wallace, M. T., 160, 187 Walter, A., 62, 100 Waiters, J. R., 62, 100 Waiters, R. H., 303, 307 Wang, L., 72, 95 Ward-Hull, C. I., 74, 90 Warfield, M. E., 247, 268 Washington, J., 250, 273 Wasserman, G. A., 246, 250, 274 Waterhouse, L., 253, 263 Waters, E., 192, 221,233, 234, 235,236, 237, 239, 240, 241,244, 260, 262, 263, 273, 274 Waters, J. M., 111, 114, 115, 117, 118, 126, 137, 138, 140, 147 Watkins, K. E., 109, 152 Watson, 253, 266 Watson, J. S., 158, 181 Watson, N. V., 74, 100 Watson, S. M., 246, 249, 268 Webb, S. J., 110, 150 Wedell-Monnig, J., 249, 274 Weiner, T., 37 Weinfeld, N. S., 234, 274 Weise, I. B., 3, 36 Weiss, B., 203,224 Weisz, J. R., 203, 205, 221,227 Wellman, B. L., 26, 38 Wendt, J., 11, 29, 37 Wenner, J. A., 107, 108, 111, 112, 113, 114, 115, 117, 118, 119, 121,124, 128, 130, 138, 139, 142, 146, 147, 152 Werner, N. E., 80, 86, 89, 92 Werth, L. H., 249, 274 Werthmann, M. W., 247, 267 West, G. B., 44, 100 West, S. G., 195, 204, 212, 225, 228 West, T. A., 105, 152 Westen, D., 71, 91 Westendorp, R. G. J., 66, 100 Wewerka, S. S., 107, 108, 111, 112, 113, 114, 115, 117, 118, 119, 121,124, 128, 130, 138, 139, 144, 147 White, J., 207, 212, 225 White, J. L., 208, 209, 229 White, S. H., 105, 151 Whitelaw, A., 248, 270 Whiting, B. B., 77, 78, 79, 81, 82, 83, 84, 93, 101

328

Author Index

Wickings, E. J., 62, 93 Widmayer, S. M., 251,267 Wiebe, S. A., 111,114, 115, 117, 118, 126, 137, 138, 140, 147 Wiemer, B. L., 193, 204, 222 Wilcox, S., 289, 310 Wiley, R. H., 57, 61, 62, 101 Wilkenson, L. K., 160, 187 Wilkins, K., 69, 87, 101 Wille, D. E., 249, 274 Willemsen-Swinkels, S. H. N., 254, 274 Williams, B. R., 201,229 Williams, E. M., 76, 94 Williams, G. C., 42, 44, 46, 48, 51, 52, 64, 101 Williams, J., 64, 70, 95 Williams, R. A., 292, 299, 308 Willingham, W. W., 83, 101 Wilson, E. O., 49, 57, 59, 97 Wilson, M., 69, 71, 72, 73, 74, 87, 92, 101 Wilson, S., 142, 152 Windle, M., 195,206, 229 Winemiller, K. O., 56, 101 Wippman, J., 235, 274 Withington-Wray, D. J., 160, 161, 187 Wittenberg, J. E, 47, 101 Wizemann, T. M., 72, 73, 74, 101 Wood, J. W., 101 Wood, W., 293,310

Woollacott, M., 25, 39, 40 Wosinski, M., 212, 223 Wrangham, R., 62, 64, 65, 70, 88, 95, 101

Yasuno, E, 110, 152 Yirmiya, N., 253, 269, 272, 274 Yonas, A., 24, 25, 38, 39, 40 Young, J., 29, 36

Zahavi, A., 55, 61, 101 Zahn-Waxler, C., 200, 208, 221,222 Zax, M., 232, 272 Zentner, M., 214, 225 Zhang, Q., 292, 309 Ziaie, H., 200, 227 Zimmerman, B. J., 284, 310 Zohari, S., 48, 98 Zola-Morgan, S., 109, 152 Zoll, D., 235,270 Zuk, M., 42, 56, 94, 101 Zumbahlen, M. R., 201, 221

Subject Index t following page number indicates a table, f following page number indicates a figure. children with sensory deficits, 249-250 chronically ill children, 250-251 Down syndrome children, 251-252, 261-264 early disorganization, 235-236 maternal sensitivity, 236-239 organization overview, 233-234 parents with psychopathology, 261 physically handicapped children, 250-251 preterm infants, 247-249 role of child characteristics, 242-243 Strange Situation, 234-235 temperament effects, 243-246 Attention control, social competence relation, resiliency as mediator, 217-218 early recall memory, 122-123 future study on development, 180 intersensory redundancy, 177 Auditory input, visual perception, 162 Autism, attachment security effects, 252-255

Active participation, infants and young children recall, 114-115 Adaptive responding crawling and walking posture flexibility, 20 sitting and crawling posture flexibility, 13-14 Adolescents, friends' influence children's reports v s . self-reports, 288t, 288-290 direction of influence, assessment, 278-281 effects in friendships, 300 influence v s . friendship selection, 286-287 peaking in middle adolescence, 291-294 similarity and group shifts, 281-283 social pressure as main process, 283-286 strength of influence, 290-291 Affirmations, definition, 132t Age effects emotion-related regulation extraorganismic v s . intraorganismic regulation, 198-202 internal mental or cognitive coping strategies, 202-203 stressor strategies, 203-204 long-term recall prevalence, 110-111 long-term recall robustness, 112-113 temperament-memory relations in 2nd year, 124-127 Alcohol abusers, mothers, attachment security effects, 255-258 Amodal relations early detection, 168 human-based studies, 171 infants, 156-158 Arousal, emotion regulation measurement, 209 Attachment security associations, 241-242 atypical children overview, 246-247 autistic children, 252-255

Balance control crawling and walking postures, 21-22 infants, 1-2 learning consequences, 7-9 local variability, 5-6 perception-action systems, 7 region of permissible postural sway, 3-4 sway region size, 5 overview, 2-3 sway model consequences, 7-9 local variability, 5-6 perception-action systems, 7 region of permissible postural sway, 3-4 sway region size, 5 329

330

Subject Index

Balance control (Continued) threat detection falling tasks, 25-27 overview, 23-24 visual cliff and locomotor experience, 24-25 BAS tendencies, emotion regulation, 211, 214 Behavioral inhibition, emotion regulation measurement, 208 BIS tendencies, emotion regulation, 211,216 Bowerbirds, behavioral intrasexual competition, 59-61 Bowlby's attachment theory, emotion regulation, 192-193 Boys' peer relationships, sex differences in human development, 77-79

CAH, see Congenital adrenal hyperplasia Chimpanzees, physical intrasexual competition, 59 Chronic illness, attachment security effects, 250-251 Cognition, emotion regulation, 192 Cognitive coping strategies, emotion-related regulation, 202-203 Colobus monkeys, physical intrasexual competition, 58 Conformity, friends' influence, 292 Congenital adrenal hyperplasia, sex differences in human development, 79 Crawling edges of gaps developmental design and rationale, 9-10 gaps apparatus, 10-12 postural flexibility and specificity, 13-16 replication, 16-17 staircase procedure, 12-13 as postural milestone, 7f slopes developmental design and rationale, 17 flexibility and specificity, 19-23 platform and procedure, 17-18 Crested newt intersexual choice, 54 male fitness, 55f Culture, phenotypic plasticity in human life history, 72t

Declarative memory, definition, 108 Deflections, definition, 132t Delayed recall, 13- and 20-month-olds, 125t Depression maternal, attachment security effects, 258-260 sex differences in human development, 80-81 Discrete emotions theory, emotion regulation, 192 Dispositional resiliency, relation to socioemotional functioning attentional control-social competence mediation, 217-218 basic relationship, 216-217 overview, 215-216 Disruptive behavior, assessment of friends' influence, 279 Down syndrome, attachment security effects, 251-252, 261-264 Drug abusers, mothers, attachment security effects, 255-258

Early Adolescent Temperament Questionnaire, emotion regulation measurement, 205-206 Early recall memory early studies overview, 105-106 elicited imitation as measure, 107-109 explicit, neural substrate, 109-110 temperament relationship, 122-123 variability sources, 120-121 EATQ, see Early Adolescent Temperament Questionnaire Ecology, sex differences in human development, 83-84 Effortful control emotion regulation, 197-198, 210-212, 219-220 socioemotional functioning, relation to dispositional resiliency attentional control-social competence mediation, 217-218 basic relationship, 216-217 overview, 215-216 Elaborations, definition, 132t

Subject Index

Elicited imitation maternal language relations, 131-133 recall measurement, 107-109 Emotional availability, maternal sensitivity, 237-238 Emotion regulation antecedent vs. concurrent, 195-196 Bowlby's attachment theory, 192-193 cognition contributions, 192 control vs. regulation, 196-197 current theories, 191-192 definition, 190-191 discrete emotions theory, 192 early theories, 191 effortful vs. reactive control, 197-198 extraorganismic vs. intraorganismic regulation, 198-202 functional theory view, 193-194 future studies, 219-220 internal mental or cognitive coping strategies, 202-203 maternal sensitivity, 237 measurement overview, 204-205 multimethod approaches, 209-210 observational measures, 207-209 parent-report measures, 206-207 physiological measures, 209 relation to social functioning quality externalizing problems, 212-214 internalizing problems, 214-215 theoretical expectations, 210-212 self-report measures, 205-206 site of regulation, 194-195 stressor strategies, 203-204 teacher-report measures, 206-207 Enabling relations, ordered recall in infants and young children, 113-114 Endocrine functions, phenotypic plasticity in human life history, 72t Everyday experience, development constraint on motor learning daily frequency of locomotion, 29-30 experience effects, 32-34 nature of travels, 30-31 overview, 27-29 quantifying steps, 31-32 Evolution intersexual choice, 67-68 intrasexual competition, 65-67 overview, 63-64

331

Explicit memory definition, 108 neural substrate, 109-110 Externalization, emotion regulation, 212-214

Facial disappointment, emotion regulation measurement, 208 Falling tasks balance threat detection, 25-27 sitting and crawling posture flexibility and specificity, 14-16 Family conflict, characteristics, 66t Family functioning, phenotypic plasticity in human life history, 72t Family-oriented play, sex differences in human development, 81-82 Female choice, characteristics, 66t Female-female competition, characteristics, 66t Flexibility crawling and walking postures adaptive responding, 20 gauging balance control, 21-22 knowledge specificity, 22-23 learning time, 20-21 overview, 19 developmental constraints on motor learning, 34-35 sitting and crawling postures adaptive responding, 13-14 falling, 14-16 Four-step sequences imitation-based studies, 107t long-term recall gender effects, 117-118 maternal language-recall performance-temperament relations, 134-136 Friendship quality differing quality, friends' characteristics effects, 299-302 high-quality, effect on children's self-esteem, 296-299, 305 judging by positive features, 294-296 Friends' influence children's reports vs. self-reports, 288t, 288-290 direction of influence, assessment, 278-281 effects in friendships, 299-302

332

Subject Index

Friends' influence (Continued) influence vs. friendship selection, 286-287 overview, 277-278 pathways of influence, 302-307 peaking in middle adolescence, 291-294 similarity and group shifts, 281-283 social pressure as main process, 283-286, 303-304 strength of influence, 290-291 Functional theory of emotion, emotion regulation, 193-194

Gaps apparatus, sitting and crawling, 10-12 Gaps studies, sitting and crawling at edge developmental design and rationale, 9-10 gaps apparatus, 10-12 postural flexibility and specificity, 13-16 replication, 16-17 staircase procedure, 12-13 Gender long-term recall effects, 116-118 recall memory variability, 139 Girls' peer relationships, sex differences in human development, 79-81 Group polarization, friends' influence, 281 Group shifts, friends' influence, 281-283 Growth, life history pattern, 49

Hippocampus, explicit memory, 109-110

Imitation-based studies, stimuli examples, 107t Implicit memory, definition, 108 Infantile amnesia, definition, 105-106 Infants balance control consequences, 7-9 importance, 1-2 local variability, 5-6 overview, 2-3 perception-action systems, 7 region of permissible postural sway, 3-4 sway region size, 5

balance threat detection falling tasks, 25-27 overview, 23-24 visual cliff and locomotor experience, 24-25 crawling and walking down slopes developmental design and rationale, 17 flexibility and specificity, 19-23 platform and procedure, 17-18 development constraint on motor learning, everyday experience daily frequency of locomotion, 29-30 experience effects, 32-34 nature of travels, 30-31 overview, 27-29 quantifying steps, 31-32 development constraint on motor learning, flexibility and specificity, 34-35 neurodevelopment relations, 140f, 143f recall, experimental manipulation, 113-115 sitting and crawling at edges of gaps developmental design and rationale, 9-10 gaps apparatus, 10-12 postural flexibility and specificity, 13-16 replication, 16-17 staircase procedure, 12-13 temperament-memory relationship, 123-124 Initial learning, long-term recall effects, 120 Insecure avoidant attachments, Strange Situation, 234 Insecure resistant attachments, Strange Situation, 234-235 Interactional synchrony, maternal sensitivity, 238 Internalization, emotion regulation, 214-215 Internal mental strategies, emotion-related regulation, 202-203 Intersensory interactions, neural and behavioral evidence, 159-163 Intersensory redundancy hypothesis, perceptual development animal-based studies, 173-175 basis for facilitation, 175-178 early development specificity, 167-169 empirical support, 169-170 human-based studies, 170-173 overview, 163-167 Intersexual choice characteristics, 66t human evolution, 67-68

333

Subject I n d e x

sexual selection, 54-56 sexual selection-life history relation, 61-62 Intrasexual competition human evolution, 65-67 sexual selection, 53-54

Language emotion-related regulation, 200-201 long-term recall effects, 118-120 maternal, see Maternal language Learning initial, long-term recall effects, 120 keep balance consequences, 7-9 local variability, 5-6 perception-action systems, 7 region of permissible postural sway, 3-4 sway region size, 5 motor, see Motor learning perceptual, intersensory redundancy, 175-178 Lekking species intersexual choice, 61 phenotypic plasticity, 62 Life history basic principles overview, 44-46 growth and development, 49 lifetime pattern of reproduction, 46-48 men's sexual selection, 73-75 phenotypic plasticity, 49-51 related traits, 64t reproductive costs, 48 sex differences life history pattern, 69-70 overview, 68 sexual selection and men's life history, 73-75 sexual selection and women's life history, 70-73 sexual selection relation behavioral intrasexual competition, 59-61 intersexual choice, 61-62 overview, 56-57 phenotypic plasticity, 62-63 physical intrasexual competition, 57-59 predicted influence, 69t women's sexual selection, 70-73

Life span, reproductive costs, 48 Locomoter experience balance threat detection, 24-25 developmental contraints on motor learning daily frequency, 29-30 overview, 27-29 Long-term recall age-related changes prevalence, 110-111 robustness, 112-113 individual differences early recall memory variability, 120-121 gender effects, 116-118 initial learning effects, 120 overview, 115-116 productive and receptive language effects, 118-120 temperament and maternal language older children's memory narratives, 131 overview, 130-131 performance relationship, 133-138 younger children elicited imitation, 131-133 temperament relationship definitions, 121-122 early recall memory, 122-123 infancy and preschool years, 123-124 2nd year, age differences, 124-127 subject differences, 127-130

M

Magnification hypothesis, friends' influence, effects in friendships, 300, 306 Male choice, characteristics, 66t Male fitness, indicators, 55f Male-male competition characteristics, 66t sexual selection-life history relation, 58 Matemal Behavior Q-Sort, maternal sensitivity, 239-240 Maternal caregiving, maternal depression effects, 259 Maternal depression, attachment security effects, 258-260 Matemal language, long-term recall, children's temperament older children's memory narratives, 131 overview, 130-131

334

Subject Index

Maternal language (Continued) performance relationship, 133-138 younger children elicited imitation, 131-133 Maternal sensitivity attachment security associations, 241-242 autistic children, 252-255 children with sensory deficits, 249-250 chronically ill children, 250-251 conceptual issues, 236-239 Down syndrome children, 251-252 drug and alcohol abusing mothers, 255-258 methodological issues, 239-241 physically handicapped children, 250-251 preterm infants, 248-249 Maternal verbal behavior memory narrative relations, 142-143 relationships in infancy, 140f, 143f Mating, parenting comparison, 51-53 Medial temporal lobe, recall memory effects, 108-109

Memory narratives maternal language relations, 131 maternal verbal behavior relations, 142-143 Middle adolescence, peaking of friends' influence, 291-294 Mnemonic abilities, relationships in infancy, 140f, 143f Mother-infant behavioral bids, maternal sensitivity, 238 Mothers, atypical, attachment security effects drug and alcohol abusers, 255-258 maternal depression, 258-260 Motor learning balance threats falling tasks, 25-27 overview, 23-24 visual cliff and locomotor experience, 24-25 crawling and walking down slopes developmental design and rationale, 17 flexibility and specificity, 19-23 platform and procedure, 17-18 development constraints, everyday experience content daily frequency of locomotion, 29-30 everyday experience, 31-34 nature of travels, 30-31 overview, 27-29 development constraints, flexibility and specificity, 34-35

sitting and crawling at edges of gaps developmental design and rationale, 9-10 gaps apparatus, 10-12 postural flexibility and specificity, 13-16 replication, 16-17 staircase procedure, 12-13 Multimodal stimulation early experience, 156-158 future study on development, 179 human-based studies, 172 intersensory redundancy, 165, 167, 176 unimodal dichotomy in developmental research, 158-159

Natural selection basic principles, 42-44 Darwin's and Wallace's Observations, 43t Negations, definition, 132t Neural pathways, intersensory interactions, 159-163 Neurodevelopment, relationships in infancy, 140f, 143f Nine-month old, long-term recall, age-related changes, 110-111 Nondeclarative memory, definition, 108 Nonverbal measurements, recall, elicited imitation as, 107-109 Northern elephant seal, physical intrasexual competition, 58

Observational methods, emotion regulation measurement, 207-209 Off-task, definition, 132t Operational sex ratio phenotypic plasticity in human life history, 72t sexual selection, 53 Ordered recall infants and young children, 113-114 1 to 12 month retention intervals, 112t OSR, see Operational sex ratio

Subject Index

Parental investment, sexual selection, 52 Parent-child relationship, overview, 231-232 Parenting, mating comparison, 51-53 Parent-offspring conflict, characteristics, 66t Parent reports, emotion regulation, 206-207 Patas monkey, physical intrasexual competition, 58 Peer relationships, sex differences in human development, 76-81 Perception-action systems, balance control, developmental changes, 7 Perceptual development future study, 178-180 historical overview, 153-156 intersensory redundancy hypothesis animal-based studies, 173-175 basis for facilitation, 175-178 early development specificity, 167-169 empirical support, 169-170 human-based studies, 170-173 overview, 163-167 Perceptual discrimination, intersensory redundancy, 175-178 Perceptual illusions, intersensory convergence, 162 Perceptual learning, intersensory redundancy, 175-178 Person-event transactions, emotion regulation, 193 Phenotypic plasticity human life history, 72t humans, 50-51 life history pattern, 49-51 sex differences in human development, 85 sexual selection-life history relation, 62-63 Physical competition, sexual selection-life history relation, 57-59 Physical handicaps, attachment security effects, 250-251 Physical model, balance control, 4f Play hunting, sex differences in human development, 84-85 Play parenting, sex differences in human development, 81-82 Play patterns, sex differences in human development, 82-85

333

Prefrontal cortex, explicit memory, 109-110 Preschool years, temperament-memory relationship, 123-124 Preterm infants, attachment security, 247-249 Productive language, long-term recall effects, 118-120

Questionnaires, maternal sensitivity, 240-241

Reactive control emotion regulation, 197-198, 219-220 socioemotional functioning, relation to dispositional resiliency attentional control-social competence mediation, 217-218 basic relationship, 216-217 overview, 215-216 Recall memory first 2 years of life developmental change effects, 110-113 infants and young children, experimental manipulation, 113-115 gender and vocabulary variability, 139 Receptive language, long-term recall effects, 118-120 Region of permissible postural sway, learning to keep balance, 3-4 Regulations, definition, 132t Relational aggression, sex differences in human development, 80-81 Repeated experience, infants and young children recall, 114 Repetitions, definition, 132t Replication, sitting and crawling at edges of gaps, 16-17 Reproduction, lifetime pattern, 46-48 Reproductive costs, life history pattern, 48 Reproductive effort, sexual selection, 51-52 Reproductive rates, sexual selection, 52-53 Reward dominance games, emotion regulation measurement, 208-209 Risky shift, friends' influence, 281

336

Subject Index

Second year of life age-related changes long-term recall prevalence, 110-111 long-term recall robustness, 112-113 age-related differences, temperament-memory relationship, 124-127 neurodevelopment relations, 14Of, 143f Secure attachments, Strange Situation, 234 Self-esteem, children's, high-quality friendship effects, 296-299, 305 Self-reports emotion regulation measurement, 205-206 friends' vs. children's reports, 288t, 288-290 Semelparity, characteristics, 46-47 Sensory deficits, effect on attachments, 249-250 Sex differences developmental activity ecologically related play patterns, 82-85 family-oriented play, 81-82 overview, 75-76 peer relationships, 76-81 play parenting, 81-82 social play, 76-81 evolution intersexual choice, 67-68 intrasexual competition, 65-67 overview, 63-64 life history life history pattern, 69-70 overview, 68 sexual selection and men's life history, 73-75 sexual selection and women's life history, 70-73 Sex hormones, phenotypic plasticity in human life history, 72t Sexual selection intersexual choice, 54-56, 67-68 intrasexual competition, 53-54, 65-67 life history relation behavioral intrasexual competition, 59-61 intersexual choice, 61-62 overview, 56-57 phenotypic plasticity, 62-63

physical intrasexual competition, 57-59 predicted influence, 69t mating vs. parenting, 51-53 men's life history, 73-75 women's life history, 70-73 Sibling conflict, characteristics, 66t Sitting edges of gaps developmental design and rationale, 9-10 gaps apparatus, 1O- 12 postural flexibility and specificity, 13-16 replication, 16-17 staircase procedure, 12-13 as postural milestone, 7f Six-step sequences, maternal language-recall performance-temperament relations, 136-137 Slopes, crawling and walking developmental design and rationale, 17 flexibility and specificity, 19-23 platform and procedure, 17-18 Social competence, attentional control relation, resiliency as mediator, 217-218 Social competition, related traits, 64t Social functioning quality, relation to emotion-related regulation externalizing problems, 212-214 internalizing problems, 214-215 theoretical expectations, 210-212 Social play, sex differences in human development, 76-81 Social pressure, role in friends' influence, 283-286, 303-304 Social segregation, sex differences in human development, 77 Social support, emotion-related regulation, 199 Socioemotional functioning, effortful and reactive control, relation to dispositional resiliency attentional control-social competence mediation, 217-218 basic relationship, 216-217 overview, 215-216 Spousal conflict, characteristics, 66t Staircase procedure, sitting and crawling at edges of gaps, 12-13 Strange Situation, attachment security, 234-235,244 Stressors, emotion-related regulation, 203-204

337

Subject Index

Sway model balance control consequences, 7-9 local variability, 5-6 perception-action systems, 7 region of permissible postural sway, 3-4 sway region size, 5 balance threat detection falling tasks, 25-27 overview, 23-24 visual cliff and locomotor experience, 24-25

Tactile perception, intersensory interactions, 162 TBAQ, see Toddler Behavior Assessment Questionnaire Teacher reports, emotion regulation, 206-207 Temperament long-term recall relationship definitions, 121-122 early recall memory, 122-123 infancy and preschool years, 123-124 2nd year, age differences, 124-127 subject differences, 127-130 long-term recall relationship, maternal language older children's memory narratives, 131 overview, 130-131 performance relationship, 133-138 younger children elicited imitation, 131-133 relationships in infancy, 140f, 143f typically developing children, 243-246 Temporal synchrony, early detection, 168 Theories of emotion, emotion regulation Bowlby's attachment theory, 192-193 cognition contributions, 192 current theories, 191-192 discrete emotions theory, 192 early theories, 191 functional view, 193-194 Threat detection, balance falling tasks, 25-27 overview, 23-24 visual cliff and locomotor experience, 24-25

Three-step sequences imitation-based studies, 107t long-term recall gender effects, 118 Toddler Behavior Assessment Questionnaire, temperament-memory relations, 124-126 Tomato hornworm, life history stages, 46f Tool use, sex differences in human development, 82-83 Two-step sequences, imitation-based studies, 107t

Unimodal relations, multimodal dichotomy in developmental research, 158-159 Unimodal stimulation future study on development, 179 human-based studies, 172 intersensory redundancy, 165-167, 169

Verbal behavior, relationships in infancy, 140f, 143f Verbal reminding, infants and young children recall, 115 Verbal report, elicited imitation as nonverbal analog, 108-109 Visual cliff balance threat detection, 24-25 sitting and crawling at edges of gaps, 10-11 Visual perception, auditory input, 162 Vocabulary long-term recall effects, 119 recall memory variability, 139

W

Walking as postural milestone, 7f slopes developmental design and rationale, 17 flexibility and specificity, 19-23 platform and procedure, 17-18 Walkway, adjustable slope, 18f Wealth, phenotypic plasticity in human life history, 72t

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Contents of Previous Volumes

Volume 1

Responses of Infants and Children to Complex and Novel Stimulation Gordon N. Cantor Word Associations and Children's Verbal Behavior David S. Palermo Change in the Stature and Body Weight of North American Boys during the Last 80 Years Howard V. Meredith Discrimination Learning Set in Children Hayne W. Reese Learning in the First Year of Life Lewis P. Lipsitt Some Methodological Contributions from a Functional Analysis of Child Development Sidney W. Bijou and Donald M. Baer The Hypothesis of Stimulus Interaction and an Explanation of Stimulus Compounding Charles C. Spiker The Development of "Overconstancy" in Space Perception Joachim E Wohlwill Miniature Experiments in the Discrimination Learning of Retardates Betty J. House and David Zeaman

Social Reinforcement of Children's Behavior Harold W. Stevenson Delayed Reinforcement Effects Glenn Terrell A Developmental Approach to Learning and Cognition Eugene S. Gollin Evidence for a Hierarchical Arrangement of Learning Processes Sheldon H. White Selected Anatomic Variables Analyzed for Interage Relationships of the Size-Size, Size-Gain, and Gain-Gain Varieties Howard V. Meredith AUTHOR INDEX--SUBJECT INDEX

Volume 3

Infant Sucking Behavior and Its Modification Herbert Kaye The Study of Brain Electrical Activity in Infants Robert J. Ellingson Selective Auditory Attention in Children Eleanor E. Maccoby Stimulus Definition and Choice Michael D. Zeiler Experimental Analysis of Inferential Behavior in Children Tracy S. Kendler and Howard H. Kendler Perceptual Integration in Children Herbert L. Pick, Jr., Anne D. Pick, and Robert E. Klein Component Process Latencies in Reaction Times of Children and Adults Raymond H. Hohle

AUTHOR INDEX--SUBJECT INDEX

Volume 2

The Paired-Associates Method in the Study of Conflict Alfred Castaneda Transfer of Stimulus Pretraining to Motor PairedAssociate and Discrimination Learning Tasks Joan H. Cantor The Role of the Distance Receptors in the Development of Social Responsiveness Richard H. Waiters and Ross D. Parke

AUTHOR INDEX~SUBJECT INDEX 339

340

Contents o f Previous Volumes

Volume 4

Developmental Studies of Figurative Perception David Elkind The Relations of Short-Term Memory to Development and Intelligence John M. Belmont and Earl C. Butterfield Learning, Developmental Research, and Individual Differences Frances Degen Horowitz Psychophysiological Studies in Newborn Infants S. J. Hutt, H. G. Lenard, and H. E R. Prechtl Development of the Sensory Analyzers during Infancy Yvonne Brackbill and Hiram E. Fitzgerald The Problem of Imitation Justin Aronfreed AUTHOR INDEX--SUBJECT INDEX

Volume 5

The Development of Human Fetal Activity and Its Relation to Postnatal Behavior Tryphena Humphrey Arousal Systems and Infant Heart Rate Responses Frances K. Graham and Jan C. Jackson Specific and Diversive Exploration Corinne Hutt Developmental Studies of Mediated Memory John H. Flavell Development and Choice Behavior in Probabilistic and Problem-Solving Tasks L. R. Goulet and Kathryn S. Goodwin AUTHOR INDEX--SUBJECT INDEX

Volume 6

Incentives and Learning in Children Sam L. Witryol Habituation in the Human Infant Wendell E. Jeffrey and Leslie B. Cohen Application of Hull-Spence Theory to the Discrimination Learning of Children Charles C. Spiker

Growth in Body Size: A Compendium of Findings on Contemporary Children Living in Different Parts of the World Howard V. Meredith Imitation and Language Development James A. Sherman Conditional Responding as a Paradigm for Observational, Imitative Learning and VicariousReinforcement Jacob L. Gewirtz AUTHOR INDEX--SUBJECT INDEX

Volume 7

Superstitious Behavior in Children: An Experimental Analysis Michael D. Zeiler Learning Strategies in Children from Different Socioeconomic Levels Jean L. Bresnahan and Martin M. Shapiro Time and Change in the Development of the Individual and Society Klaus E Riegel The Nature and Development of Early Number Concepts Rochel Gelman Learning and Adaptation in Infancy: A Comparison of Models Arnold J. Sameroff AUTHOR INDEX--SUBJECT INDEX

Volume 8

Elaboration and Learning in Childhood and Adolescence William D. Rohwer, Jr. Exploratory Behavior and Human Development Jum C. Nunnally and L. Charles Lemond Operant Conditioning of Infant Behavior: A Review Robert C. Hulsebus Birth Order and Parental Experience in Monkeys and Man G. Mitchell and L. Schroers Fear of the Stranger: A Critical Examination Harriet L. Rheingold and Carol O. Eckerman

Contents of Previous Volumes

Applications of Hull-Spence Theory to the Transfer of Discrimination Learning in Children Charles C. Spiker and Joan H. Cantor

AUTHOR INDEX--SUBJECT INDEX

Volume 9

Children's Discrimination Learning Based on Identity or Difference Betty J. House, Ann L. Brown, and Marcia S. Scott

Two Aspects of Experience in Ontogeny: Development and Learning Hans G. Furth

The Effects of Contextual Changes and Degree of Component Mastery on Transfer of Training Joseph C. Campione and Ann L. Brown

Psychophysiological Functioning, Arousal, Attention, and Learning during the First Year of Life Richard Hirschman and Edward S. Katkin

Self-Reinforcement Process in Children John C. Masters and Janice R. Mokros

AUTHOR INDEX--SUBJECT INDEX

341

Volume 11

The Hyperactive Child: Characteristics, Treatment, and Evaluation of Research Design Gladys B. Baxley and Judith M. LeBlanc

Peripheral and Neurochemical Parallels of Psychopathology: A Psychophysiological Model Relating Autonomic Imbalance to Hyperactivity, Psychopathy, and Autism Stephen W. Porges

Constructing Cognitive Operations Linguistically Harry Beilin

Operant Acquisition of Social Behaviors in Infancy: Basic Problems and Constraints W. Stuart Millar

Mother-Infant Interaction and Its Study Jacob L. Gewirtz and Elizabeth E Boyd

Symposium on Implications of Life-Span Developmental Psychology for Child Development: Introductory Remarks Paul B. Baltes

Theory and Method in Life-Span Developmental Psychology: Implications for Child Development Aletha Huston-Stein and Paul B. Baltes

The Development of Memory: Life-Span Perspectives Hayne W. Reese

Volume 10

Current Trends in Developmental Psychology Boyd R. McCandless and Mary Fulcher Geis

The Development of Spatial Representations of Large-Scale Environments Alexander W. Siegel and Sheldon H. White

Cognitive Perspectives on the Development of Memory John W. Hagen, Robert H. Jongeward, Jr., and Robert V. Kail, Jr.

The Development of Memory: Knowing, Knowing About Knowing, and Knowing How to Know Ann L. Brown

Cognitive Changes during the Adult Years: Implications for Developmental Theory and Research Nancy W. Denney and John C. Wright

Social Cognition and Life-Span Approaches to the Study of Child Development Michael J. Chandler

Life-Span Development of the Theory of Oneself: Implications for Child Development Orville G. Brim, Jr.

Implications of Life-Span Developmental Psychology for Childhood Education Leo Montada and Sigrun-Heide Filipp

AUTHOR INDEX--SUBJECT INDEX

Developmental Trends in Visual Scanning Mary Carol Day

The Development of Selective Attention: From Perceptual Exploration to Logical Search John C. Wright and Alice G. Vlietstra

AUTHOR INDEX--SUBJECT INDEX

Volume 12

Research between 1960 and 1970 on the Standing Height of Young Children in Different Parts of the World Howard V. Meredith

342

Contents of Previous Volumes

The Representation of Children's Knowledge David Klahr and Robert S. Siegler Chromatic Vision in Infancy Marc H. Bornstein Developmental Memory Theories: Baldwin and Piaget Bruce M. Ross and Stephen M. Kerst Child Discipline and the Pursuit of Self: An Historical Interpretation Howard Gadlin Development of Time Concepts in Children William J. Friedman

Sibship-Constellation Effects on Psychosocial Development, Creativity, and Health Mazie Earle Wagner, Herman J. P. Schubert, and Daniel S. P. Schubert The Development of Understanding of the Spatial Terms Front and Back Lauren Julius Harris and Ellen A. Strommen The Organization and Control of Infant Sucking C. K. Crook Neurological Plasticity, Recovery from Brain Insult, and Child Development lan St. James-Roberts

AUTHOR INDEX--SUBJECT INDEX

AUTHOR INDEX--SUBJECT INDEX

Volume 13

Coding of Spatial and Temporal Information in Episodic Memory Daniel B. Berch A Developmental Model of Human Learning Barry Gholson and Harry Beilin The Development of Discrimination Learning: A Levels-of-Functioning Explanation Tracy S. Kendler The Kendler Levels-of-Functioning Theory: Comments and an Alternative Schema Charles C. Spiker and Joan H. Cantor Commentary on Kendler's Paper: An Alternative Perspective Barry GhoIson and Therese Schuepfer Reply to Commentaries Tracy S. Kendler On the Development of Speech Perception: Mechanisms and Analogies Peter D. Eimas and Vivien C. Tartter The Economics of Infancy: A Review of Conjugate Reinforcement Carolyn Kent Rovee-Collier and Marcy J. Gekoski Human Facial Expressions in Response to Taste and Smell Stimulation Jacob E. Steiner AUTHOR INDEX--SUBJECT INDEX Volume 14

Development of Visual Memory in Infants John S. Werner and Marion Perlmutter

Volume 15

Visual Development in Ontogenesis: Some Reevaluations Jiiri Allik and Jaan Valsiner Binocular Vision in Infants: A Review and a Theoretical Framework Richard N. Aslin and Susan T. Dumais Validating Theories of Intelligence Earl C. Butterfield, Dennis Siladi, and John M. Belmont Cognitive Differentiation and Developmental Learning William Fowler Children's Clinical Syndromes and Generalized Expectations of Control Fred Rothbaum AUTHOR INDEX--SUBJECT INDEX

Volume 16

The History of the Boyd R. McCandless Young Scientist Awards: The First Recipients David Palermo Social Bases of Language Development: A Reassessment Elizabeth Bates, Inge Bretherton, Marjorie Beeghly-Smith, and Sandra McNew Perceptual Anisotrophies in Infancy: Ontogenetic Origins and Implications of Inequalities in Spatial Vision Marc H. Bornstein

Contents of Previous Volumes

Concept Development Martha J. Farah and Stephen M. Kosslyn Production and Perception of Facial Expressions in Infancy and Early Childhood Tiffany M. Field and Tedra A. Walden Individual Differences in Infant Sociability: Their Origins and Implications for Cognitive Development Michael E. Lamb The Development of Numerical Understandings Robert S. Siegler and Mitchell Robinson AUTHOR INDEXmSUBJECT INDEX

Volume 17

The Development of Problem-Solving Strategies Deanna Kuhn and Erin Phelps Information Processing and Cognitive Development Robert Kail and Jeffrey Bisanz Research between 1950 and 1980 on Urban-Rural Differences in Body Size and Growth Rate of Children and Youths Howard V. Meredith Word Meaning Acquisition in Young Children: A Review of Theory and Research Pamela Blewitt Language Play and Language Acquisition Stan A. Kuczaj H The Child Study Movement: Early Growth and Development of the Symbolized Child Alexander W. Siegel and Sheldon H. White AUTHOR INDEX--SUBJECT INDEX

Volume 18

The Development of Verbal Communicative Skills in Children Constance R. Schmidt and Scott G. Paris Auditory Feedback and Speech Development Gerald M. Siegel, Herbert L. Pick, Jr., and Sharon R. Garber Body Size of Infants and Children around the World in Relation to Socioeconomic Status Howard V. Meredith Human Sexual Dimorphism: Its Cost and Benefit James L. Mosley and Eileen A. Stan

343

Symposium on Research Programs: Rational Alternatives to Kuhn's Analysis of Scientific Progress--Introductory Remarks Hayne W. Reese, Chairman World Views and Their Influence on Psychological Theory and Research: Kuhn-Lakatos-Laudan Willis E Overton The History of the Psychology of Learning as a Rational Process: Lakatos versus Kuhn Peter Barker and Barry Gholson Functionalist and Structuralist Research Programs in Developmental Psychology: Incommensurability or Synthesis? Harry Beilin In Defense of Kuhn: A Discussion of His Detractors David S. Palermo Comments on Beilin's Epistemology and Palermo's Defense of Kuhn Willis F. Overton From Kuhn to Lakatos to Laudan Peter Barker and Barry Gholson Overton's and Palermo's Relativism: One Step Forward, Two Steps Back Harry Beilin AUTHOR INDEX--SUBJECT INDEX

Volume 19

Response to Novelty: Continuity versus Discontinuity in the Developmental Course of Intelligence Cynthia A. Berg and Robert J. Sternberg Metaphoric Competence in Cognitive and Language Development Marc Marschark and Lynn Nall The Concept of Dimensions in Developmental Research Stuart I. Offenbach and Francine C. Blumberg Effects of the Knowledge Base on Children's Memory Strategies Peter A. Ornstein and Mary J. Naus Effects of Sibling Spacing on Intelligence, Interfamilial Relations, Psychosocial Characteristics, and Mental and Physical Health Mazie Earle Wagner, Herman J. P. Schubert, and Daniel S. P. Schubert

344

Contents of Previous Volumes

Infant Visual Preferences: A Review and New Theoretical Treatment Martin S. Banks and Arthur P. Ginsburg AUTHOR INDEX--SUBJECT INDEX

Volume 20

Variation in Body Stockiness among and within Ethnic Groups at Ages from Birth to Adulthood Howard V. Meredith The Development of Conditional Reasoning: An Iffy Proposition David P.. O'Brien Content Knowledge: Its Role, Representation, and Restructuring in Memory Development Michelene T. H. Chi and Stephen J. Ceci Descriptions: A Model of Nonstrategic Memory Development Brian P. Ackerman Reactivation of Infant Memory: Implications for Cognitive Development Carolyn Rovee-Collier and Harlene Hayne Gender Segregation in Childhood Eleanor E. Maccoby and Carol Nagy Jacklin Piaget, Attentional Capacity, and the Functional Implications of Formal Structure Michael Chapman INDEX

Volume 21

Social Development in Infancy: A 25-Year Perspective Ross D. Parke On the Uses of the Concept of Normality in Developmental Biology and Psychology Eugene S. Gollin, Gary Stahl, and Elyse Morgan Cognitive Psychology: Mentalistic or Behavioristic? Charles C. Spiker Some Current Issues in Children's Selective Attention Betty J. House Children's Learning Revisited: The Contemporary Scope of the Modified Spence Discrimination Theory Joan H. Cantor and Charles C. Spiker

Discrimination Learning Set in Children Hayne W Reese A Developmental Analysis of Rule-Following Henry C. Riegler and Donald M. Baer Psychological Linguistics: Implications for a Theory of Initial Development and a Method for Research Sidney W Bijou Psychic Conflict and Moral Development Gordon N. Cantor and David A. Parton Knowledge and the Child's Developing Theory of the World David S. Palermo Childhood Events Recalled by Children and Adults David B. Pillemer and Sheldon H. White INDEX

Volume 22

The Development of Representation in Young Children Judy S. DeLoache Children's Understanding of Mental Phenomena David Estes, Henry M. Wellman, and Jacqueline D. Woolley Social Influences on Children's Cognition: State of the Art and Future Directions Margarita Azmitia and Marion Perlmutter Understanding Maps as Symbols: The Development of Map Concepts Lynn S. Liben and Roger M. Downs The Development of Spatial Perspective Taking Nora Newcombe Developmental Studies of Alertness and Encoding Effects of Stimulus Repetition Daniel W Smothergill and Alan G. Kraut Imitation in Infancy: A Critical Review Claire L. Poulson, Leila Regina de Paula Nunes, and Steven E Warren AUTHOR INDEXmSUBJECT INDEX

Volume 23

The Structure of Developmental Theory Willis E Overton

Contents of Previous Volumes

Questions a Satisfying Developmental Theory Would Answer: The Scope of a Complete Explanation of Development Phenomena Frank B. Murray The Development of World Views: Toward Future Synthesis? Ellin Kofsky Scholnick Metaphor, Recursive Systems, and Paradox in Science and Developmental Theory Willis E Overton Children's Iconic Realism: Object versus Property Realism Harry Beilin and Elise G. Pearlman The Role of Cognition in Understanding Gender Effects Carol Lynn Martin Development of Processing Speed in Childhood and Adolescence Robert Kail Contextualism and Developmental Psychology Hayne W. Reese Horizontality of Water Level: A Neo-Piagetian Developmental Review Juan Pascual-Leone and Sergio Morra AUTHOR INDEX--SUBJECT INDEX

Volume 24

Music and Speech Processing in the First Year of Life Sandra E. Trehub, Laurel J. Trainor, and Anna M. Unyk Effects of Feeding Method on Infant Temperament John Worobey The Development of Reading Linda S. Siegel Learning to Read: A Theoretical Synthesis John P. Rack, Charles Hulme, and Margaret J. Snowling Does Reading Make You Smarter? Literacy and the Development of Verbal Intelligence Keith E. Stanovich Sex-of-Sibling Effects: Part I. Gender Role, Intelligence, Achievement, and Creativity Mazie Earle Wagner, Herman J. P. Schubert, and Daniel S. P. Schubert The Concept of Same Linda B. Smith

345

Planning as Developmental Process Jacquelyn Baker-Sennett, Eugene Matusov, and Barbara Rogoff AUTHOR INDEX--SUBJECT INDEX

Volume 25

In Memoriam: Charles C. Spiker (1925-1993) Lewis P Lipsitt Developmental Differences in Associative Memory: Strategy Use, Mental Effort, and Knowledge Access Interactions Daniel W. Kee A Unifying Framework for the Development of Children's Activity Memory Hilary Horn Ratner and Mary Ann Foley Strategy Utilization Deficiencies in Children: When, Where, and Why Patricia H. Miller and Wendy L. Seier The Development of Children's Ability to Use Spatial Representations Mark Blades and Christopher Spencer Fostering Metacognitive Development Linda Baker The HOME Inventory: Review and Reflections Robert H. Bradley Social Reasoning and the Varieties of Social Experiences in Cultural Contexts Elliot Turiel and Cecilia Wainryb Mechanisms in the Explanation of Developmental Change Harry Beilin AUTHOR INDEX--SUBJECT INDEX

Volume 26

Preparing to Read: The Foundations of Literacy Ellen Bialystok The Role of Schemata in Children's Memory Denise Davidson The Interaction of Knowledge, Aptitude, and Strategies in Children's Memory Performance David E Bjorklund and Wolfgang Schneider Analogical Reasoning and Cognitive Development Usha Goswami

346

Contents of Previous Volumes

Sex-of-Sibling Effects: A Review Part II. Personality and Mental and Physical Health Mazie Earle Wagner, Herman J. P. Schubert, and Daniel S. P. Schubert Input and Learning Processes in First Language Acquisition Ernst L. Moerk AUTHOR INDEX--SUBJECT INDEX

Volume 27

From Form to Meaning: A Role for Structural Alignment in the Acquisition of Language Cynthia Fisher The Role of Essentialism in Children's Concepts Susan A. Gelman Infants' Use of Prior Experiences with Objects in Object Segregation: Implications for Object Recognition in Infancy Amy Needham and Avani Modi Perseveration and Problem Solving in Infancy Andrba Aguiar and Renbe Baillargeon Temperament and Attachment: One Construct or Two? Sarah C. Mangelsdorf and Cynthia A. Frosch The Foundation of Piaget's Theories: Mental and Physical Action Harry Beilin and Gary Fireman AUTHOR INDEX--SUBJECT INDEX

Volume 28

Variability in Children's Reasoning Karl S. Rosengren and Gregory S. Braswell Fuzzy-Trace Theory: Dual Processes in Memory, Reasoning, and Cognitive Neuroscience C. J. Brainerd and V. E Reyna

Relational Frame Theory: A Post-Skinnerian Account of Human Language and Cognition Yvonne Barnes-Holmes, Steven C. Hayes, Dermot Barnes-Holmes, and Bryan Roche The Continuity of Depression across the Adolescent Transition Shelli Avenevoli and Laurence Steinberg The Time of Our Lives: Self-Continuity in Native and Non-Native Youth Michael J. Chandler AUTHOR INDEXwSUBJECT INDEX

Volume 29

The Search for What is Fundamental in the Development of Working Memory Nelson Cowan, J. Scott Saults, and Emily M. Elliott Culture, Autonomy, and Personal Jurisdiction in Adolescent-Parent Relationships Judith G. Smetana Maternal Responsiveness and Early Language Acquisition Catherine S. Tamis-Lemonda and Marc H. Bornstein Schooling as Cultural Process: Working Together and Guidance by Children from Schools Differing in Collaborative Practices Eugene Matusov, Nancy Bell, and Barbara Rogoff Beyond Prototypes: Asymmetries in Infant Categorization and What They Teach Us about the Mechanisms Guiding Early Knowledge Acquisition Paul C. Quinn Peer Relations in the Transition to Adolescence Carollee Howes and Julie Wargo Aikins AUTHOR INDEX---SUBJECT INDEX